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There are six essentials in painting. The first is called spirit; the second, rhythm; the third, thought; the fourth, scenery; the fifth, the brush; and the last is the ink.
Ching Hao
The previous chapter provided an overview of PL/SQL. This chapter focuses on the small-scale aspects of the language. Like every other programming language, PL/SQL has a character set, reserved words, punctuation, datatypes, rigid syntax, and fixed rules of usage and statement formation. You use these basic elements of PL/SQL to represent real-world objects and operations.
You write a PL/SQL program as lines of text using a specific set of characters. The PL/SQL character set includes
PL/SQL is not case sensitive, so lowercase letters are equivalent to corresponding uppercase letters except within string and character literals.
A line of PL/SQL text contains groups of characters known as lexical units, which can be classified as follows:
For example, the line
bonus := salary * 0.10; -- compute bonus
contains the following lexical units:
bonus and salary
:=
* and ;
0.10
-- compute bonus
To improve readability, you can separate lexical units by spaces. In fact, you must separate adjacent identifiers by a space or punctuation. The following line is illegal because the reserved words END and IF are joined:
IF x > y THEN high := x; ENDIF; -- illegal
However, you cannot embed spaces in lexical units except for string literals and comments. For example, the following line is illegal because the compound symbol for assignment (:=) is split:
count : = count + 1; -- illegal
To show structure, you can divide lines using carriage returns and indent lines using spaces or tabs. Compare these IF statements for readability:
IF x>y THEN max:=x;ELSE max:=y;END IF; | IF x > y THEN | max := x; | ELSE | max := y; | END IF;
A delimiter is a simple or compound symbol that has a special meaning to PL/SQL. For example, you use delimiters to represent arithmetic operations such as addition and subtraction.
Simple symbols consist of one character; a list follows:
Compound symbols consist of one character; a list follows:
You use identifiers to name PL/SQL program items and units, which include constants, variables, exceptions, cursors, cursor variables, subprograms, and packages. Some examples of identifiers follow:
X t2 phone# credit_limit LastName oracle$number
An identifier consists of a letter optionally followed by more letters, numerals, dollar signs, underscores, and number signs. Other characters such as hyphens, slashes, and spaces are illegal, as the following examples show:
mine&yours -- illegal ampersand debit-amount -- illegal hyphen on/off -- illegal slash user id -- illegal space
The next examples show that adjoining and trailing dollar signs, underscores, and number signs are legal:
money$$$tree SN## try_again_
You can use upper, lower, or mixed case to write identifiers. PL/SQL is not case sensitive except within string and character literals. So, if the only difference between identifiers is the case of corresponding letters, PL/SQL considers the identifiers to be the same, as the following example shows:
lastname LastName -- same as lastname LASTNAME -- same as lastname and LastName
The length of an identifier cannot exceed 30 characters. But, every character, including dollar signs, underscores, and number signs, is significant. For example, PL/SQL considers the following identifiers to be different:
lastname last_name
Identifiers should be descriptive. So, avoid obscure names such as cpm. Instead, use meaningful names such as cost_per_thousand.
Some identifiers, called reserved words, have a special syntactic meaning to PL/SQL and so should not be redefined. For example, the words BEGIN and END, which bracket the executable part of a block or subprogram, are reserved. As the next example shows, if you try to redefine a reserved word, you get a compilation error:
DECLARE end BOOLEAN; -- illegal; causes compilation error
However, you can embed reserved words in an identifier, as the following example shows:
DECLARE end_of_game BOOLEAN; -- legal
Often, reserved words are written in upper case to promote readability. However, like other PL/SQL identifiers, reserved words can be written in lower or mixed case. For a list of reserved words, see Appendix F.
Identifiers globally declared in package STANDARD, such as the exception INVALID_NUMBER, can be redeclared. However, redeclaring predefined identifiers is error prone because your local declaration overrides the global declaration.
For flexibility, PL/SQL lets you enclose identifiers within double quotes. Quoted identifiers are seldom needed, but occasionally they can be useful. They can contain any sequence of printable characters including spaces but excluding double quotes. Thus, the following identifiers are legal:
"X+Y" "last name" "on/off switch" "employee(s)" "*** header info ***"
The maximum length of a quoted identifier is 30 characters not counting the double quotes. Though allowed, using PL/SQL reserved words as quoted identifiers is a poor programming practice.
Some PL/SQL reserved words are not reserved by SQL. For example, you can use the PL/SQL reserved word TYPE in a CREATE TABLE statement to name a database column. But, if a SQL statement in your program refers to that column, you get a compilation error, as the following example shows:
SELECT acct, type, bal INTO ... -- causes compilation error
To prevent the error, enclose the uppercase column name in double quotes, as follows:
SELECT acct, "TYPE", bal INTO ...
The column name cannot appear in lower or mixed case (unless it was defined that way in the CREATE TABLE statement). For example, the following statement is invalid:
SELECT acct, "type", bal INTO ... -- causes compilation error
Alternatively, you can create a view that renames the troublesome column, then use the view instead of the base table in SQL statements.
A literal is an explicit numeric, character, string, or Boolean value not represented by an identifier. The numeric literal 147 and the Boolean literal FALSE are examples.
Two kinds of numeric literals can be used in arithmetic expressions: integers and reals. An integer literal is an optionally signed whole number without a decimal point. Some examples follow:
030 6 -14 0 +32767
A real literal is an optionally signed whole or fractional number with a decimal point. Several examples follow:
6.6667 0.0 -12.0 3.14159 +8300.00 .5 25.
PL/SQL considers numbers such as 12.0 and 25. to be reals even though they have integral values.
Numeric literals cannot contain dollar signs or commas, but can be written using scientific notation. Simply suffix the number with an E (or e) followed by an optionally signed integer. A few examples follow:
2E5 1.0E-7 3.14159e0 -1E38 -9.5e-3
E stands for "times ten to the power of." As the next example shows, the number after E is the power of ten by which the number before E must be multiplied (the double asterisk (**) is the exponentiation operator):
5E3 = 5 10**3 = 5 1000 = 5000
The number after E also corresponds to the number of places the decimal point shifts. In the last example, the implicit decimal point shifted three places to the right; in the next example, it shifts three places to the left:
5E-3 = 5 10**-3 = 5 0.001 = 0.005
A character literal is an individual character enclosed by single quotes (apostrophes). Several examples follow:
'Z' '%' '7' ' ' 'z' '('
Character literals include all the printable characters in the PL/SQL character set: letters, numerals, spaces, and special symbols.
PL/SQL is case sensitive within character literals. For example, PL/SQL considers the literals 'Z' and 'z' to be different.
The character literals '0'..'9' are not equivalent to integer literals, but can be used in arithmetic expressions because they are implicitly convertible to integers.
A character value can be represented by an identifier or explicitly written as a string literal, which is a sequence of zero or more characters enclosed by single quotes. Several examples follow:
'Hello, world!' 'XYZ Corporation' '10-NOV-91' 'He said "Life is like licking honey from a thorn."' '$1,000,000'
All string literals except the null string ('') have datatype CHAR.
Given that apostrophes (single quotes) delimit string literals, how do you represent an apostrophe within a string? As the next example shows, you write two single quotes, which is not the same as writing a double quote:
'Don''t leave without saving your work.'
PL/SQL is case sensitive within string literals. For example, PL/SQL considers the following literals to be different:
'baker' 'Baker'
Boolean literals are the predefined values TRUE and FALSE and the non-value NULL, which stands for a missing, unknown, or inapplicable value. Remember, Boolean literals are values, not strings. For example, TRUE is no less a value than the number 25.
The PL/SQL compiler ignores comments, but you should not. Adding comments to your program promotes readability and aids understanding. Generally, you use comments to describe the purpose and use of each code segment. PL/SQL supports two comment styles: single-line and multi-line.
Single-line comments begin with a double hyphen (--) anywhere on a line and extend to the end of the line. A few examples follow:
-- begin processing SELECT sal INTO salary FROM emp -- get current salary WHERE empno = emp_id; bonus := salary * 0.15; -- compute bonus amount
Notice that comments can appear within a statement at the end of a line.
While testing or debugging a program, you might want to disable a line of code. The following example shows how you can "comment-out" the line:
-- DELETE FROM emp WHERE comm IS NULL;
Multi-line comments begin with a slash-asterisk (/*), end with an asterisk-slash (*/), and can span multiple lines. Some examples follow:
BEGIN /* Compute a 15% bonus for top-rated employees. */ IF rating > 90 THEN bonus := salary * 0.15 /* bonus is based on salary */ ELSE bonus := 0; END If; ... /* The following line computes the area of a circle using pi, which is the ratio between the circumference and diameter. */ area := pi * radius**2;
You can use multi-line comment delimiters to comment-out whole sections of code, as the following example shows:
/* LOOP FETCH c1 INTO emp_rec; EXIT WHEN c1%NOTFOUND; ... END LOOP; */
You cannot nest comments. Also, you cannot use single-line comments in a PL/SQL block that will be processed dynamically by an Oracle Precompiler program because end-of-line characters are ignored. As a result, single-line comments extend to the end of the block, not just to the end of a line. So, use multi-line comments instead.
Every constant and variable has a datatype, which specifies a storage format, constraints, and valid range of values. PL/SQL provides a variety of predefined datatypes. A scalar type has no internal components. A composite type has internal components that can be manipulated individually. A reference type holds values, called pointers, that designate other program items. A LOB type holds values, called locators, that specify the location of large objects (graphic images for example) stored out-of-line.
Figure 2-1 shows the predefined datatypes available for your use. An additional scalar type, MLSLABEL, is available with Trusted Oracle, a specially secured version of Oracle. The scalar types fall into four families, which store number, character, Boolean, and date/time data, respectively.
This section discusses the scalar types and LOB types. The composite types are discussed in Chapter 4. The reference types are discussed in Chapter 5 and Chapter 9.
Number types allow you to store numeric data (integers, real numbers, and floating-point numbers), represent quantities, and do calculations.
You use the BINARY_INTEGER datatype to store signed integers. Its magnitude range is -2147483647 .. 2147483647. Like PLS_INTEGER values, BINARY_INTEGER values require less storage than NUMBER values. However, most BINARY_INTEGER operations are slower than PLS_INTEGER operations. (See "PLS_INTEGER".)
A base type is the datatype from which a subtype is derived. A subtype associates a base type with a constraint and so defines a subset of values. For your convenience, PL/SQL predefines the following BINARY_INTEGER subtypes:
The subtypes NATURAL and POSITIVE let you restrict an integer variable to non-negative or positive values, respectively. NATURALN and POSITIVEN prevent the assigning of nulls to an integer variable. SIGNTYPE lets you restrict an integer variable to the values -1, 0, and 1, which is useful in programming tri-state logic.
You use the NUMBER datatype to store fixed-point or floating-point numbers of virtually any size. Its magnitude range is 1.0E-130 .. 9.99E125. You can specify precision, which is the total number of digits, and scale, which is the number of digits to the right of the decimal point. The syntax follows:
NUMBER[(precision,scale)]
To declare fixed-point numbers, for which you must specify scale, use the following form:
NUMBER(precision,scale)
To declare floating-point numbers, for which you cannot specify precision or scale because the decimal point can "float" to any position, use the following form:
NUMBER
To declare integers, which have no decimal point, use this form:
NUMBER(precision) -- same as NUMBER(precision,0)
You cannot use constants or variables to specify precision and scale; you must use integer literals. The maximum precision of a NUMBER value is 38 decimal digits. If you do not specify precision, it defaults to 38 or the maximum supported by your system, whichever is less.
Scale, which can range from -84 to 127, determines where rounding occurs. For instance, a scale of 2 rounds to the nearest hundredth (3.456 becomes 3.46). A negative scale rounds to the left of the decimal point. For example, a scale of -3 rounds to the nearest thousand (3456 becomes 3000). A scale of 0 rounds to the nearest whole number. If you do not specify scale, it defaults to 0.
You can use the following NUMBER subtypes for compatibility with ANSI/ISO and IBM types or when you want a more descriptive name:
Use the subtypes DEC, DECIMAL, and NUMERIC to declare fixed-point numbers with a maximum precision of 38 decimal digits.
Use the subtypes DOUBLE PRECISION and FLOAT to declare floating-point numbers with a maximum precision of 126 binary digits, which is roughly equivalent to 38 decimal digits. Or, use the subtype REAL to declare floating-point numbers with a maximum precision of 63 binary digits, which is roughly equivalent to 18 decimal digits.
Use the subtypes INTEGER, INT, and SMALLINT to declare integers with a maximum precision of 38 decimal digits.
You use the PLS_INTEGER datatype to store signed integers. Its magnitude range is -2147483647 .. 2147483647. PLS_INTEGER values require less storage than NUMBER values. Also, PLS_INTEGER operations use machine arithmetic, so they are faster than NUMBER and BINARY_INTEGER operations, which use library arithmetic. For better performance, use PLS_INTEGER for all calculations that fall within its magnitude range.
Although PLS_INTEGER and BINARY_INTEGER have the same magnitude range, they are not fully compatible. When a PLS_INTEGER calculation overflows, an exception is raised. However, when a BINARY_INTEGER calculation overflows, no exception is raised if the result is assigned to a NUMBER variable.
Because of this small semantic difference, you might want to continue using BINARY_INTEGER in old applications for compatibility. In new applications, always use PLS_INTEGER for better performance.
Character types allow you to store alphanumeric data, represent words and text, and manipulate character strings.
You use the CHAR datatype to store fixed-length character data. How the data is represented internally depends on the database character set, which might be 7-bit ASCII or EBCDIC Code Page 500, for example.
The CHAR datatype takes an optional parameter that lets you specify a maximum length up to 32767 bytes. The syntax follows:
CHAR[(maximum_length)]
You cannot use a constant or variable to specify the maximum length; you must use an integer literal in the range 1 .. 32767.
If you do not specify a maximum length, it defaults to 1. Remember, you specify the maximum length in bytes, not characters. So, if a CHAR(n) variable stores multi-byte characters, its maximum length is less than n characters. The maximum width of a CHAR database column is 2000 bytes. So, you cannot insert CHAR values longer than 2000 bytes into a CHAR column.
You can insert any CHAR(n) value into a LONG database column because the maximum width of a LONG column is 2147483647 bytes or 2 gigabytes. However, you cannot retrieve a value longer than 32767 bytes from a LONG column into a CHAR(n) variable.
The CHAR subtype CHARACTER has the same range of values as its base type. That is, CHARACTER is just another name for CHAR. You can use this subtype for compatibility with ANSI/ISO and IBM types or when you want an identifier more descriptive than CHAR.
You use the LONG datatype to store variable-length character strings. The LONG datatype is like the VARCHAR2 datatype, except that the maximum length of a LONG value is 32760 bytes.
You can insert any LONG value into a LONG database column because the maximum width of a LONG column is 2147483647 bytes. However, you cannot retrieve a value longer than 32760 bytes from a LONG column into a LONG variable.
LONG columns can store text, arrays of characters, or even short documents. You can reference LONG columns in UPDATE, INSERT, and (most) SELECT statements, but not in expressions, SQL function calls, or certain SQL clauses such as WHERE, GROUP BY, and CONNECT BY. For more information, see Oracle8 SQL Reference.
You use the RAW datatype to store binary data or byte strings. For example, a RAW variable might store a sequence of graphics characters or a digitized picture. Raw data is like VARCHAR2 data, except that PL/SQL does not interpret raw data. Likewise, Net8 does no character set conversions when you transmit raw data from one system to another.
The RAW datatype takes a required parameter that lets you specify a maximum length up to 32767 bytes. The syntax follows:
RAW(maximum_length)
You cannot use a constant or variable to specify the maximum length; you must use an integer literal in the range 1 .. 32767.
The maximum width of a RAW database column is 2000 bytes. So, you cannot insert RAW values longer than 2000 bytes into a RAW column.
You can insert any RAW value into a LONG RAW database column because the maximum width of a LONG RAW column is 2147483647 bytes. However, you cannot retrieve a value longer than 32767 bytes from a LONG RAW column into a RAW variable.
You use the LONG RAW datatype to store binary data or byte strings. LONG RAW data is like LONG data, except that LONG RAW data is not interpreted by PL/SQL. The maximum length of a LONG RAW value is 32760 bytes.
You can insert any LONG RAW value into a LONG RAW database column because the maximum width of a LONG RAW column is 2147483647 bytes. However, you cannot retrieve a value longer than 32760 bytes from a LONG RAW column into a LONG RAW variable.
Internally, every database table has a ROWID pseudocolumn, which stores hexadecimal strings called rowids. Each rowid represents the storage address of a row. You use the ROWID datatype to store rowids.
You can compare a ROWID variable with the ROWID pseudocolumn in the WHERE clause of an UPDATE or DELETE statement to identify the latest row fetched from a cursor. See "Fetching Across Commits".
With Oracle8, rowids have been extended to support partitioned tables and indexes. Extended rowids include a data object number, which identifies the database segment. Schema objects in the same segment (for example, a cluster of tables) have the same object number.
A rowid contains the following information, which is needed to locate a row:
Rowids provide the fastest way to access particular rows. Normally, a rowid uniquely identifies a row. However, rows in different tables stored in the same cluster can have the same rowid.
To manipulate rowids, you use the supplied package DBMS_ROWID, which is described in Oracle8 Application Developer's Guide.
You use the VARCHAR2 datatype to store variable-length character data. How the data is represented internally depends on the database character set, which might be 7-bit ASCII or EBCDIC Code Page 500, for example.
The VARCHAR2 datatype takes a required parameter that specifies a maximum length up to 32767 bytes. The syntax follows:
VARCHAR2(maximum_length)
You cannot use a constant or variable to specify the maximum length; you must use an integer literal in the range 1 .. 32767.
Remember, you specify the maximum length of a VARCHAR2(n) variable in bytes, not characters. So, if a VARCHAR2(n) variable stores multi-byte characters, its maximum length is less than n characters. The maximum width of a VARCHAR2 database column is 4000 bytes. Therefore, you cannot insert VARCHAR2 values longer than 4000 bytes into a VARCHAR2 column.
You can insert any VARCHAR2(n) value into a LONG database column because the maximum width of a LONG column is 2147483647 bytes. However, you cannot retrieve a value longer than 32767 bytes from a LONG column into a VARCHAR2(n) variable.
Semantic differences between the CHAR and VARCHAR2 base types are discussed in Appendix C.
The VARCHAR2 subtypes below have the same range of values as their base type. For example, VARCHAR is just another name for VARCHAR2.
You can use these subtypes for compatibility with ANSI/ISO and IBM types.
Although the widely used 7- or 8-bit ASCII and EBCDIC character sets are adequate to represent the Roman alphabet, some Asian languages, such as Japanese, contain thousands of characters. These languages require 16 bits (two bytes) to represent each character. How does Oracle deal with such dissimilar languages?
Oracle provides National Language Support (NLS), which lets you process single-byte and multi-byte character data and convert between character sets. It also lets your applications run in different language environments.
With NLS, number and date formats adapt automatically to the language conventions specified for a user session. Thus, NLS allows users around the world to interact with Oracle in their native languages. For more information about NLS, see Oracle8 SQL Reference.
PL/SQL V2 supports just one character set called the database character set, which is used for identifiers and source code. But, PL/SQL V8 supports a second character set called the national character set, which is used for NLS data. The datatypes NCHAR and NVARCHAR2 store character strings formed from the national character set.
You use the NCHAR datatype to store fixed-length (blank-padded if necessary) NLS character data. How the data is represented internally depends on the national character set, which might use a fixed-width encoding such as US7ASCII or a variable-width encoding such as JA16SJIS.
The NCHAR datatype takes an optional parameter that lets you specify a maximum length up to 32767 bytes. The syntax follows:
NCHAR[(maximum_length)]
You cannot use a constant or variable to specify the maximum length; you must use an integer literal in the range 1 .. 32767.
If you do not specify a maximum length, it defaults to 1. How you specify the maximum length depends on the national character set. For fixed-width character sets, you specify the maximum length in characters. For variable-width character sets, you specify it in bytes. In the following example, the character set is JA16EUCFIXED (which is fixed-width), so you specify the maximum length in characters:
my_string NCHAR(100); -- maximum length is 100 characters
The maximum width of an NCHAR database column is 2000 bytes. So, you cannot insert NCHAR values longer than 2000 bytes into an NCHAR column. Remember, for fixed-width, multi-byte character sets, you cannot insert NCHAR values longer than the number of characters that fit in 2000 bytes.
If the NCHAR value is shorter than the defined width of the NCHAR column, Oracle blank-pads the value to the defined width. You cannot insert CHAR values into an NCHAR column. Likewise, you cannot insert NCHAR values into a CHAR column.
You use the NVARCHAR2 datatype to store variable-length NLS character data. How the data is represented internally depends on the national character set, which might use a fixed-width encoding such as WE8EBCDIC37C or a variable-width encoding such as JA16DBCS.
The NVARCHAR2 datatype takes a required parameter that specifies a maximum length up to 32767 bytes. The syntax follows:
NVARCHAR2(maximum_length)
You cannot use a constant or variable to specify the maximum length; you must use an integer literal in the range 1 .. 32767.
How you specify the maximum length depends on the national character set. For fixed-width character sets, you specify the maximum length in characters. For variable-width character sets, you specify it in bytes. In the following example, the character set is JA16SJIS (which is variable-width), so you specify the maximum length in bytes:
my_string NVARCHAR2(200); -- maximum length is 200 bytes
The maximum width of a NVARCHAR2 database column is 4000 bytes. Therefore, you cannot insert NVARCHAR2 values longer than 4000 bytes into a NVARCHAR2 column. Remember, for fixed-width, multi-byte character sets, you cannot insert NVARCHAR2 values longer than the number of characters that fit in 4000 bytes.
You cannot insertVARCHAR2 values into an NVARCHAR2 column. Likewise, you cannot insert NVARCHAR2 values into a VARCHAR2 column.
The LOB (large object) datatypes BFILE, BLOB, CLOB, and NCLOB let you store blocks of unstructured data (such as text, graphic images, video clips, and sound waveforms) up to four gigabytes in size. And, they allow efficient, random, piece-wise access to the data.
The LOB types differ from the LONG and LONG RAW types in several ways. For example, LOBs (except NCLOB) can be attributes of an object type, but LONGs cannot. The maximum size of a LOB is four gigabytes, but the maximum size of a LONG is two gigabytes. Also, LOBs support random access to data, but LONGs support only sequential access.
LOB types store values, called locators, that specify the location of large objects stored in an external file, in-line (inside the row) or out-of-line (outside the row). Database columns of type BLOB, CLOB, NCLOB, or BFILE store the locators. BLOB, CLOB, and NCLOB data is stored in the database, in or outside the row. BFILE data is stored in operating system files outside the database.
PL/SQL operates on LOBs through the locators. For example, when you retrieve a BLOB column value, only a locator is returned. Locators cannot span transactions or sessions. So, you cannot save a locator in a PL/SQL variable during one transaction or session, then use it in another transaction or session. To manipulate LOBs, you use the supplied package DBMS_LOB. For more information about LOBs and package DBMS_LOB, see Oracle8 Application Developer's Guide.
You use the BFILE datatype to store large binary objects in operating system files outside the database. Every BFILE variable stores a file locator, which points to a large binary file on the server. The locator includes a directory alias, which specifies a full path name (logical path names are not supported).
BFILEs are read-only. You cannot modify them. The maximum number of open BFILEs is set by the Oracle initialization parameter SESSION_MAX_OPEN_FILES, which is system dependent.
Also, BFILEs do not participate in transactions. The underlying operating system maintains file integrity. The size of a BFILE is system dependent but cannot exceed four gigabytes (2**32 - 1 bytes). Your DBA makes sure that the BFILE exists and that Oracle has read permissions on it.
You use the BLOB datatype to store large binary objects in the database in-line or out-of-line. Every BLOB variable stores a locator, which points to a large binary object. The size of a BLOB cannot exceed four gigabytes.
BLOBs participate fully in transactions. Changes made by package DBMS_LOB or the OCI can be committed or rolled back. However, BLOB locators cannot span transactions or sessions.
You use the CLOB datatype to store large blocks of single-byte character data in the database, in-line or out-of-line. (Variable-width character sets are not supported.) Every CLOB variable stores a locator, which points to a large block of character data. The size of a CLOB cannot exceed four gigabytes.
CLOBs participate fully in transactions. Changes made by package DBMS_LOB or the OCI can be committed or rolled back. However, CLOB locators cannot span transactions or sessions.
You use the NCLOB datatype to store large blocks of single-byte or fixed-width multi-byte NCHAR data in the database, in-line or out-of-line. (Variable-width character sets are not supported.) Every NCLOB variable stores a locator, which points to a large block of NCHAR data. The size of an NCLOB cannot exceed four gigabytes.
NCLOBs participate fully in transactions. Changes made by package DBMS_LOB or the OCI can be committed or rolled back. However, NCLOB locators cannot span transactions or sessions.
The following types allow you to store and manipulate logical (true, false) values, date/time values, and Trusted Oracle operating system labels.
You use the BOOLEAN datatype to store the logical values TRUE and FALSE and the non-value NULL, which stands for a missing, inapplicable, or unknown value. Only logic operations are allowed on BOOLEAN variables.
The BOOLEAN datatype takes no parameters. Only the values TRUE and FALSE and the non-value NULL can be assigned to a BOOLEAN variable. You cannot insert the values TRUE and FALSE into a database column. Also, you cannot select or fetch column values into a BOOLEAN variable.
You use the DATE datatype to store fixed-length date/time values. DATE values include the time of day in seconds since midnight. The date portion defaults to the first day of the current month; the time portion defaults to midnight. The date function SYSDATE returns the current date and time.
Valid dates range from January 1, 4712 BC to December 31, 4712 AD. A Julian date is the number of days since January 1, 4712 BC. Julian dates allow continuous dating from a common reference. You can use the date format model 'J' with date functions TO_DATE and TO_CHAR to convert between DATE values and their Julian equivalents.
In date expressions, PL/SQL automatically converts character values in the default date format to DATE values. The default date format is set by the Oracle initialization parameter NLS_DATE_FORMAT. For example, the default might be 'DD-MON-YY', which includes a two-digit number for the day of the month, an abbreviation of the month name, and the last two digits of the year.
You can add and subtract dates. For example, the following statement returns the number of days since an employee was hired:
SELECT SYSDATE - hiredate INTO days_worked FROM emp WHERE empno = 7499;
In arithmetic expressions, PL/SQL interprets integer literals as days. For instance, SYSDATE + 1 is tomorrow.
For more information about date functions and format models, see Oracle8 SQL Reference.
Trusted Oracle provides the MLSLABEL datatype, which stores Trusted Oracle's internal representation of labels generated by multi-level secure operating systems. Trusted Oracle uses the labels to control database access.
For compatibility with Trusted Oracle applications, the Oracle server lets you use MLSLABEL to define database columns. However, such columns can store only nulls. For more information, see your Trusted Oracle documentation.
Each PL/SQL base type specifies a set of values and a set of operations applicable to items of that type. Subtypes specify the same set of operations as their base type but only a subset of its values. Thus, a subtype does not introduce a new type; it merely places an optional constraint on its base type.
PL/SQL predefines several subtypes in package STANDARD. For example, PL/SQL predefines the subtype CHARACTER, as follows:
SUBTYPE CHARACTER IS CHAR;
The subtype CHARACTER specifies the same set of values as its base type CHAR. Thus, CHARACTER is an unconstrained subtype.
Subtypes can increase reliability, provide compatibility with ANSI/ISO types, and improve readability by indicating the intended use of constants and variables.
You can define your own subtypes in the declarative part of any PL/SQL block, subprogram, or package using the syntax
SUBTYPE subtype_name IS base_type;
where subtype_name is a type specifier used in subsequent declarations and base_type is any scalar or user-defined PL/SQL type. To specify base_type, you can use %TYPE, which provides the datatype of a variable or database column. Also, you can use %ROWTYPE, which provides the rowtype of a cursor, cursor variable, or database table. Some examples follow:
DECLARE SUBTYPE EmpDate IS DATE; -- based on DATE type SUBTYPE Counter IS NATURAL; -- based on NATURAL subtype TYPE NameList IS TABLE OF VARCHAR2(10); SUBTYPE EmpRoster IS NameList; -- based on TABLE type TYPE TimeRec IS RECORD (minutes INTEGER, hours INTEGER); SUBTYPE Time IS TimeRec; -- based on RECORD type SUBTYPE ID_Num IS emp.empno%TYPE; -- based on column type CURSOR c1 IS SELECT * FROM dept; SUBTYPE DeptFile IS c1%ROWTYPE; -- based on cursor rowtype
However, you cannot specify a constraint on the base type. For example, the following definitions are illegal:
DECLARE SUBTYPE Accumulator IS NUMBER(7,2); -- illegal; must be NUMBER SUBTYPE Delimiter IS CHAR(1); -- illegal; must be CHAR SUBTYPE Word IS VARCHAR2(15); -- illegal
Although you cannot define constrained subtypes directly, you can use a simple workaround to define size-constrained subtypes indirectly. Simply declare a size-constrained variable, then use %TYPE to provide its datatype, as shown in the following example:
DECLARE temp VARCHAR2(15); SUBTYPE Word IS temp%TYPE; -- maximum size of Word is 15
Likewise, if you define a subtype using %TYPE to provide the datatype of a database column, the subtype adopts the size constraint (if any) of the column. However, the subtype does not adopt other kinds of constraints such as NOT NULL.
Once you define a subtype, you can declare items of that type. In the example below, you declare two variables of type Counter. Notice how the subtype name indicates the intended use of the variables.
DECLARE SUBTYPE Counter IS NATURAL; rows Counter; employees Counter;
The following example shows that you can constrain a user-defined subtype when declaring variables of that type:
DECLARE SUBTYPE Accumulator IS NUMBER; total Accumulator(7,2);
Subtypes can increase reliability by detecting out-of-range values. In the example below, you restrict the subtype Scale to storing integers in the range -9 .. 9. If your program tries to store a number outside that range in a Scale variable, PL/SQL raises an exception.
DECLARE temp NUMBER(1,0); SUBTYPE Scale IS temp%TYPE; x_axis Scale; -- magnitude range is -9 .. 9 y_axis Scale; BEGIN x_axis := 10; -- raises VALUE_ERROR
An unconstrained subtype is interchangeable with its base type. For example, given the following declarations, the value of amount can be assigned to total without conversion:
DECLARE SUBTYPE Accumulator IS NUMBER; amount NUMBER(7,2); total Accumulator; BEGIN ... total := amount;
Different subtypes are interchangeable if they have the same base type. For instance, given the following declarations, the value of finished can be assigned to debugging:
DECLARE SUBTYPE Sentinel IS BOOLEAN; SUBTYPE Switch IS BOOLEAN; finished Sentinel; debugging Switch; BEGIN ... debugging := finished;
Different subtypes are also interchangeable if their base types are in the same datatype family. For example, given the following declarations, the value of verb can be assigned to sentence:
DECLARE SUBTYPE Word IS CHAR; SUBTYPE Text IS VARCHAR2; verb Word; sentence Text; BEGIN ... sentence := verb;
Sometimes it is necessary to convert a value from one datatype to another. For example, if you want to examine a rowid, you must convert it to a character string. PL/SQL supports both explicit and implicit (automatic) datatype conversion.
To convert values from one datatype to another, you use built-in functions. For example, to convert a CHAR value to a DATE or NUMBER value, you use the function TO_DATE or TO_NUMBER, respectively. Conversely, to convert a DATE or NUMBER value to a CHAR value, you use the function TO_CHAR. For more information about these functions, see Oracle8 SQL Reference.
When it makes sense, PL/SQL can convert the datatype of a value implicitly. This allows you to use literals, variables, and parameters of one type where another type is expected. In the example below, the CHAR variables start_time and finish_time hold string values representing the number of seconds past midnight. The difference between those values must be assigned to the NUMBER variable elapsed_time. So, PL/SQL converts the CHAR values to NUMBER values automatically.
DECLARE start_time CHAR(5); finish_time CHAR(5); elapsed_time NUMBER(5); BEGIN /* Get system time as seconds past midnight. */ SELECT TO_CHAR(SYSDATE,'SSSSS') INTO start_time FROM sys.dual; -- do something /* Get system time again. */ SELECT TO_CHAR(SYSDATE,'SSSSS') INTO finish_time FROM sys.dual; /* Compute elapsed time in seconds. */ elapsed_time := finish_time - start_time; INSERT INTO results VALUES (elapsed_time, ...); END;
Before assigning a selected column value to a variable, PL/SQL will, if necessary, convert the value from the datatype of the source column to the datatype of the variable. This happens, for example, when you select a DATE column value into a VARCHAR2 variable.
Likewise, before assigning the value of a variable to a database column, PL/SQL will, if necessary, convert the value from the datatype of the variable to the datatype of the target column. If PL/SQL cannot determine which implicit conversion is needed, you get a compilation error. In such cases, you must use a datatype conversion function. Table 2-1 shows which implicit conversions PL/SQL can do.
| BIN_INT | CHAR | DATE | LONG | NUMBER | PLS_INT | RAW | ROWID | VARCHAR2 | |
|---|---|---|---|---|---|---|---|---|---|
| BIN_INT |
|
X |
|
X |
X |
X |
|
|
X |
| CHAR |
X |
|
X |
X |
X |
X |
X |
X |
X |
| DATE |
|
X |
|
X |
|
|
|
|
X |
| LONG |
|
X |
|
|
|
|
X |
|
X |
| NUMBER |
X |
X |
|
X |
|
X |
|
|
X |
| PLS_INT |
X |
X |
|
X |
X |
|
|
|
X |
| RAW |
|
X |
|
X |
|
|
|
|
X |
| ROWID |
|
X |
|
|
|
|
|
|
X |
| VARCHAR2 |
X |
X |
X |
X |
X |
X |
X |
X |
|
It is your responsibility to ensure that values are convertible. For instance, PL/SQL can convert the CHAR value '02-JUN-92' to a DATE value but cannot convert the CHAR value 'YESTERDAY' to a DATE value. Similarly, PL/SQL cannot convert a VARCHAR2 value containing alphabetic characters to a NUMBER value.
Generally, to rely on implicit datatype conversions is a poor programming practice because they can hamper performance and might change from one software release to the next. Also, implicit conversions are context sensitive and therefore not always predictable. Instead, use datatype conversion functions. That way, your applications will be more reliable and easier to maintain.
When you select a DATE column value into a CHAR or VARCHAR2 variable, PL/SQL must convert the internal binary value to a character value. So, PL/SQL calls the function TO_CHAR, which returns a character string in the default date format. To get other information such as the time or Julian date, you must call TO_CHAR with a format mask.
A conversion is also necessary when you insert a CHAR or VARCHAR2 value into a DATE column. So, PL/SQL calls the function TO_DATE, which expects the default date format. To insert dates in other formats, you must call TO_DATE with a format mask.
When you select a RAW or LONG RAW column value into a CHAR or VARCHAR2 variable, PL/SQL must convert the internal binary value to a character value. In this case, PL/SQL returns each binary byte of RAW or LONG RAW data as a pair of characters. Each character represents the hexadecimal equivalent of a nibble (half a byte). For example, PL/SQL returns the binary byte 11111111 as the pair of characters 'FF'. The function RAWTOHEX does the same conversion.
A conversion is also necessary when you insert a CHAR or VARCHAR2 value into a RAW or LONG RAW column. Each pair of characters in the variable must represent the hexadecimal equivalent of a binary byte. If either character does not represent the hexadecimal equivalent of a nibble, PL/SQL raises an exception.
When passed an uppercase character set name, the built-in function NLS_CHARSET_ID returns the corresponding character set ID number. Conversely, when passed a character set ID number, the function NLS_CHARSET_NAME returns the corresponding character set name.
If you pass the value 'CHAR_CS' or 'NCHAR_CS' to NLS_CHARSET_ID, it returns the database or national character set ID number, respectively. For a list of character set names, see Oracle8 Reference.
Your program stores values in variables and constants. As the program executes, the values of variables can change, but the values of constants cannot.
You can declare variables and constants in the declarative part of any PL/SQL block, subprogram, or package. Declarations allocate storage space for a value, specify its datatype, and name the storage location so that you can reference it.
A couple of examples follow:
birthday DATE; emp_count SMALLINT := 0;
The first declaration names a variable of type DATE. The second declaration names a variable of type SMALLINT and uses the assignment operator to assign an initial value of zero to the variable.
The next examples show that the expression following the assignment operator can be arbitrarily complex and can refer to previously initialized variables:
pi REAL := 3.14159; radius REAL := 1; area REAL := pi * radius**2;
By default, variables are initialized to NULL. For example, the following declarations are equivalent:
birthday DATE; birthday DATE := NULL;
In constant declarations, the keyword CONSTANT must precede the type specifier, as the following example shows:
credit_limit CONSTANT REAL := 5000.00;
This declaration names a constant of type REAL and assigns an initial (also final) value of 5000 to the constant. A constant must be initialized in its declaration. Otherwise, you get a compilation error when the declaration is elaborated. (The processing of a declaration by the PL/SQL compiler is called elaboration.)
You can use the keyword DEFAULT instead of the assignment operator to initialize variables. For example, the declarations
blood_type CHAR := 'O'; valid BOOLEAN := FALSE;
can be rewritten as follows:
blood_type CHAR DEFAULT 'O'; valid BOOLEAN DEFAULT FALSE;
Use DEFAULT for variables that have a typical value. Use the assignment operator for variables (such as counters and accumulators) that have no typical value. A couple of examples follow:
hours_worked INTEGER DEFAULT 40; employee_count INTEGER := 0;
You can also use DEFAULT to initialize subprogram parameters, cursor parameters, and fields in a user-defined record.
Besides assigning an initial value, declarations can impose the NOT NULL constraint, as the following example shows:
acct_id INTEGER(4) NOT NULL := 9999;
You cannot assign nulls to a variable defined as NOT NULL. If you try, PL/SQL raises the predefined exception VALUE_ERROR. The NOT NULL constraint must be followed by an initialization clause. For example, the following declaration is illegal:
acct_id INTEGER(5) NOT NULL; -- illegal; not initialized
Recall that the subtypes NATURALN and POSITIVEN are predefined as NOT NULL. For instance, the following declarations are equivalent:
emp_count NATURAL NOT NULL := 0; emp_count NATURALN := 0;
In NATURALN and POSITIVEN declarations, the type specifier must be followed by an initialization clause. Otherwise, you get a compilation error. For example, the following declaration is illegal:
line_items POSITIVEN; -- illegal; not initialized
The %TYPE attribute provides the datatype of a variable or database column. In the following example, %TYPE provides the datatype of a variable:
credit REAL(7,2); debit credit%TYPE;
Variables declared using %TYPE are treated like those declared using a datatype specifier. For example, given the previous declarations, PL/SQL treats debit like a REAL(7,2) variable. The next example shows that a %TYPE declaration can include an initialization clause:
balance NUMBER(7,2); minimum_balance balance%TYPE := 10.00;
The %TYPE attribute is particularly useful when declaring variables that refer to database columns. You can reference a table and column, or you can reference an owner, table, and column, as in
my_dname scott.dept.dname%TYPE;
Using %TYPE to declare my_dname has two advantages. First, you need not know the exact datatype of dname. Second, if the database definition of dname changes, the datatype of my_dname changes accordingly at run time.
Note, however, that a NOT NULL column constraint does not apply to variables declared using %TYPE. In the next example, even though the database column empno is defined as NOT NULL, you can assign a null to the variable my_empno:
DECLARE my_empno emp.empno%TYPE; ... BEGIN my_empno := NULL; -- this works
The %ROWTYPE attribute provides a record type that represents a row in a table (or view). The record can store an entire row of data selected from the table or fetched from a cursor or strongly typed cursor variable. In the example below, you declare two records. The first record stores a row selected from the emp table. The second record stores a row fetched from cursor c1.
DECLARE emp_rec emp%ROWTYPE; CURSOR c1 IS SELECT deptno, dname, loc FROM dept; dept_rec c1%ROWTYPE;
Columns in a row and corresponding fields in a record have the same names and datatypes. In the following example, you select column values into a record named emp_rec:
DECLARE emp_rec emp%ROWTYPE; ... BEGIN SELECT * INTO emp_rec FROM emp WHERE ...
The column values returned by the SELECT statement are stored in fields. To reference a field, you use dot notation. For example, you might reference the deptno field as follows:
IF emp_rec.deptno = 20 THEN ...
Also, you can assign the value of an expression to a specific field, as the following examples show:
emp_rec.ename := 'JOHNSON'; emp_rec.sal := emp_rec.sal * 1.15;
In the final example, you use %ROWTYPE to define a packaged cursor:
CREATE PACKAGE emp_actions AS /* Declare cursor specification. */ CURSOR c1 RETURN emp%ROWTYPE; ... END emp_actions; CREATE PACKAGE BODY emp_actions AS /* Define cursor body. */ CURSOR c1 RETURN emp%ROWTYPE IS SELECT * FROM emp WHERE sal > 3000; ... END emp_actions;
A %ROWTYPE declaration cannot include an initialization clause. However, there are two ways to assign values to all fields in a record at once. First, PL/SQL allows aggregate assignment between entire records if their declarations refer to the same table or cursor. For example, the following assignment is legal:
DECLARE dept_rec1 dept%ROWTYPE; dept_rec2 dept%ROWTYPE; CURSOR c1 IS SELECT deptno, dname, loc FROM dept; dept_rec3 c1%ROWTYPE; BEGIN ... dept_rec1 := dept_rec2;
However, because dept_rec2 is based on a table and dept_rec3 is based on a cursor, the following assignment is illegal:
dept_rec2 := dept_rec3; -- illegal
Second, you can assign a list of column values to a record by using the SELECT or FETCH statement, as the example below shows. The column names must appear in the order in which they were defined by the CREATE TABLE or CREATE VIEW statement.
DECLARE dept_rec dept%ROWTYPE; ... BEGIN SELECT deptno, dname, loc INTO dept_rec FROM dept WHERE deptno = 30;
However, you cannot assign a list of column values to a record by using an assignment statement. So, the following syntax is illegal:
record_name := (value1, value2, value3, ...); -- illegal
Although you can retrieve entire records, you cannot insert or update them. For example, the following statement is illegal:
INSERT INTO dept VALUES (dept_rec); -- illegal
Select-list items fetched from a cursor associated with %ROWTYPE must have simple names or, if they are expressions, must have aliases. In the following example, you use an alias called wages:
-- available online in file 'examp4' DECLARE CURSOR my_cursor IS SELECT sal + NVL(comm, 0) wages, ename FROM emp; my_rec my_cursor%ROWTYPE; BEGIN OPEN my_cursor; LOOP FETCH my_cursor INTO my_rec; EXIT WHEN my_cursor%NOTFOUND; IF my_rec.wages > 2000 THEN INSERT INTO temp VALUES (NULL, my_rec.wages, my_rec.ename); END IF; END LOOP; CLOSE my_cursor; END;
PL/SQL does not allow forward references. You must declare a variable or constant before referencing it in other statements, including other declarative statements. For example, the following declaration of maxi is illegal:
maxi INTEGER := 2 * mini; -- illegal mini INTEGER := 15;
However, PL/SQL does allow the forward declaration of subprograms. For more information, see "Forward Declarations".
Some languages allow you to declare a list of variables that have the same datatype. PL/SQL does not allow this. For example, the following declaration is illegal:
i, j, k SMALLINT; -- illegal
The legal version follows:
i SMALLINT; j SMALLINT; k SMALLINT;
The same naming conventions apply to all PL/SQL program items and units including constants, variables, cursors, cursor variables, exceptions, procedures, functions, and packages. Names can be simple, qualified, remote, or both qualified and remote. For example, you might use the procedure name raise_salary in any of the following ways:
raise_salary(...); -- simple emp_actions.raise_salary(...); -- qualified raise_salary@newyork(...); -- remote emp_actions.raise_salary@newyork(...); -- qualified and remote
In the first case, you simply use the procedure name. In the second case, you must qualify the name using dot notation because the procedure is stored in a package called emp_actions. In the third case, using the remote access indicator (@), you reference the database link newyork because the procedure is stored in a remote database. In the fourth case, you qualify the procedure name and reference a database link.
You can create synonyms to provide location transparency for remote schema objects such as tables, sequences, views, stand-alone subprograms, and packages. However, you cannot create synonyms for items declared within subprograms or packages. That includes constants, variables, cursors, cursor variables, exceptions, and packaged procedures.
Within the same scope, all declared identifiers must be unique. So, even if their datatypes differ, variables and parameters cannot share the same name. For example, two of the following declarations are illegal:
DECLARE valid_id BOOLEAN; valid_id VARCHAR2(5); -- illegal duplicate identifier FUNCTION bonus (valid_id IN INTEGER) RETURN REAL IS ... -- illegal triplicate identifier
For the scoping rules that apply to identifiers, see "Scope and Visibility".
Like other identifiers, the names of constants, variables, and parameters are not case sensitive. For instance, PL/SQL considers the following names to be the same:
DECLARE zip_code INTEGER; Zip_Code INTEGER; -- same as zip_code ZIP_CODE INTEGER; -- same as zip_code and Zip_Code
In potentially ambiguous SQL statements, the names of local variables and formal parameters take precedence over the names of database tables. For example, the following UPDATE statement fails because PL/SQL assumes that emp refers to the loop counter:
FOR emp IN 1..5 LOOP ... UPDATE emp SET bonus = 500 WHERE ... END LOOP;
Likewise, the following SELECT statement fails because PL/SQL assumes that emp refers to the formal parameter:
PROCEDURE calc_bonus (emp NUMBER, bonus OUT REAL) IS avg_sal REAL; BEGIN SELECT AVG(sal) INTO avg_sal FROM emp WHERE ...
In such cases, you can prefix the table name with a username, as follows, but a better programming practice is to rename the variable or formal parameter:
PROCEDURE calc_bonus (emp NUMBER, bonus OUT REAL) IS avg_sal REAL; BEGIN SELECT AVG(sal) INTO avg_sal FROM scott.emp WHERE ...
Unlike the names of tables, the names of columns take precedence over the names of local variables and formal parameters. For example, the following DELETE statement removes all employees from the emp table, not just `KING', because Oracle assumes that both enames in the WHERE clause refer to the database column:
DECLARE ename VARCHAR2(10) := 'KING'; BEGIN DELETE FROM emp WHERE ename = ename;
In such cases, to avoid ambiguity, prefix the names of local variables and formal parameters with my_, as follows:
DECLARE my_ename VARCHAR2(10);
Or, use a block label to qualify references, as in
<<main>> DECLARE ename VARCHAR2(10) := 'KING'; BEGIN DELETE FROM emp WHERE ename = main.ename;
The next example shows that you can use a subprogram name to qualify references to local variables and formal parameters:
FUNCTION bonus (deptno IN NUMBER, ...) RETURN REAL IS job CHAR(10); BEGIN SELECT ... WHERE deptno = bonus.deptno AND job = bonus.job;
For a full discussion of name resolution, see Appendix E.
References to an identifier are resolved according to its scope and visibility. The scope of an identifier is that region of a program unit (block, subprogram, or package) from which you can reference the identifier. An identifier is visible only in the regions from which you can reference the identifier using an unqualified name. Figure 2-2 shows the scope and visibility of a variable named x, which is declared in an enclosing block, then redeclared in a sub-block.
Identifiers declared in a PL/SQL block are considered local to that block and global to all its sub-blocks. If a global identifier is redeclared in a sub-block, both identifiers remain in scope. Within the sub-block, however, only the local identifier is visible because you must use a qualified name to reference the global identifier.
Although you cannot declare an identifier twice in the same block, you can declare the same identifier in two different blocks. The two items represented by the identifier are distinct, and any change in one does not affect the other. However, a block cannot reference identifiers declared in other blocks at the same level because those identifiers are neither local nor global to the block.
The example below illustrates the scope rules. Notice that the identifiers declared in one sub-block cannot be referenced in the other sub-block. That is because a block cannot reference identifiers declared in other blocks nested at the same level.
DECLARE a CHAR; b REAL; BEGIN -- identifiers available here: a (CHAR), b DECLARE a INTEGER; c REAL; BEGIN -- identifiers available here: a (INTEGER), b, c END; DECLARE d REAL; BEGIN -- identifiers available here: a (CHAR), b, d END; -- identifiers available here: a (CHAR), b END;
Recall that global identifiers can be redeclared in a sub-block, in which case the local declaration prevails and the sub-block cannot reference the global identifier unless you use a qualified name. The qualifier can be the label of an enclosing block, as the following example shows:
<<outer>> DECLARE birthdate DATE; BEGIN DECLARE birthdate DATE; BEGIN ... IF birthdate = outer.birthdate THEN ...
As the next example shows, the qualifier can also be the name of an enclosing subprogram:
PROCEDURE check_credit (...) IS rating NUMBER; FUNCTION valid (...) RETURN BOOLEAN IS rating NUMBER; BEGIN ... IF check_credit.rating < 3 THEN ...
However, within the same scope, a label and a subprogram cannot have the same name.
Variables and constants are initialized every time a block or subprogram is entered. By default, variables are initialized to NULL. So, unless you expressly initialize a variable, its value is undefined, as the following example shows:
DECLARE count INTEGER; ... BEGIN count := count + 1; -- assigns a null to count
The expression on the right of the assignment operator yields NULL because count is null. To avoid unexpected results, never reference a variable before you assign it a value.
You can use assignment statements to assign values to a variable. For example, the following statement assigns a new value to the variable bonus, overwriting its old value:
bonus := salary * 0.15;
The expression following the assignment operator can be arbitrarily complex, but it must yield a datatype that is the same as or convertible to the datatype of the variable.
Only the values TRUE and FALSE and the non-value NULL can be assigned to a Boolean variable. For example, given the declaration
DECLARE done BOOLEAN;
the following statements are legal:
BEGIN done := FALSE; WHILE NOT done LOOP ... END LOOP;
When applied to an expression, the relational operators return a Boolean value. So, the following assignment is legal:
done := (count > 500);
Alternatively, you can use the SELECT statement to have Oracle assign values to a variable. For each item in the select list, there must be a corresponding, type-compatible variable in the INTO list. An example follows:
DECLARE my_empno emp.empno%TYPE; my_ename emp.ename%TYPE; wages NUMBER(7,2); BEGIN ... SELECT ename, sal + comm INTO last_name, wages FROM emp WHERE empno = emp_id;
However, you cannot select column values into a Boolean variable.
Expressions are constructed using operands and operators. An operand is a variable, constant, literal, or function call that contributes a value to an expression. An example of a simple arithmetic expression follows:
-X / 2 + 3
Unary operators such as the negation operator (-) operate on one operand; binary operators such as the division operator (/) operate on two operands. PL/SQL has no ternary operators.
The simplest expressions consist of a single variable, which yields a value directly. PL/SQL evaluates (finds the current value of) an expression by combining the values of the operands in ways specified by the operators. This always yields a single value and datatype. PL/SQL determines the datatype by examining the expression and the context in which it appears.
The operations within an expression are done in a particular order depending on their precedence (priority). Table 2-2 shows the default order of operations from first to last (top to bottom).
Operators with higher precedence are applied first. For example, both of the following expressions yield 8 because division has a higher precedence than addition:
5 + 12 / 4 12 / 4 + 5
Operators with the same precedence are applied in no particular order.
You can use parentheses to control the order of evaluation. For example, the following expression yields 7, not 11, because parentheses override the default operator precedence:
(8 + 6) / 2
In the next example, the subtraction is done before the division because the most deeply nested subexpression is always evaluated first:
100 + (20 / 5 + (7 - 3))
The following example shows that you can always use parentheses to improve readability, even when they are not needed:
(salary * 0.05) + (commission * 0.25)
The logical operators AND, OR, and NOT follow the tri-state logic shown in Table 2-3. AND and OR are binary operators; NOT is a unary operator.
As the truth table shows, AND returns TRUE only if both its operands are true. On the other hand, OR returns TRUE if either of its operands is true. NOT returns the opposite value (logical negation) of its operand. For example, NOT TRUE returns FALSE.
NOT NULL returns NULL because nulls are indeterminate. It follows that if you apply the NOT operator to a null, the result is also indeterminate. Be careful. Nulls can cause unexpected results; see "Handling Nulls".
When you do not use parentheses to specify the order of evaluation, operator precedence determines the order. Compare the following expressions:
NOT (valid AND done) | NOT valid AND done
If the Boolean variables valid and done have the value FALSE, the first expression yields TRUE. However, the second expression yields FALSE because NOT has a higher precedence than AND; therefore, the second expression is equivalent to
(NOT valid) AND done
In the following example, notice that when valid has the value FALSE, the whole expression yields FALSE regardless of the value of done:
valid AND done
Likewise, in the next example, when valid has the value TRUE, the whole expression yields TRUE regardless of the value of done:
valid OR done
When evaluating a logical expression, PL/SQL uses short-circuit evaluation. That is, PL/SQL stops evaluating the expression as soon as the result can be determined. This allows you to write expressions that might otherwise cause an error. Consider the following OR expression:
DECLARE ... on_hand INTEGER; on_order INTEGER; BEGIN .. IF (on_hand = 0) OR (on_order / on_hand < 5) THEN ... END IF; END;
When the value of on_hand is zero, the left operand yields TRUE, so PL/SQL need not evaluate the right operand. If PL/SQL were to evaluate both operands before applying the OR operator, the right operand would cause a division by zero error. In any case, it is a poor programming practice to rely on short-circuit evaluation.
Comparison operators compare one expression to another. The result is always TRUE, FALSE, or NULL. Typically, you use comparison operators in the WHERE clause of SQL data manipulation statements and in conditional control statements.
The relational operators allow you to compare arbitrarily complex expressions. The following list gives the meaning of each operator:
| Operator | Meaning |
|---|---|
|
= |
equal to |
|
<>, !=, ~= |
not equal to |
|
< |
less than |
|
> |
greater than |
|
<= |
less than or equal to |
|
>= |
greater than or equal to |
The IS NULL operator returns the Boolean value TRUE if its operand is null or FALSE if it is not null. Comparisons involving nulls always yield NULL. Therefore, to test for nullity (the state of being null), do not use the statement
IF variable = NULL THEN ...
Instead, use the following statement:
IF variable IS NULL THEN ...
You use the LIKE operator to compare a character value to a pattern. Case is significant. LIKE returns the Boolean value TRUE if the character patterns match or FALSE if they do not match.
The patterns matched by LIKE can include two special-purpose characters called wildcards. An underscore (_) matches exactly one character; a percent sign (%) matches zero or more characters. For example, if the value of ename is 'JOHNSON', the following expression yields TRUE:
ename LIKE 'J%SON'
The BETWEEN operator tests whether a value lies in a specified range. It means "greater than or equal to low value and less than or equal to high value." For example, the following expression yields FALSE:
45 BETWEEN 38 AND 44
The IN operator tests set membership. It means "equal to any member of.
The set can contain nulls, but they are ignored. For example, the following statement does not delete rows in which the ename column is null:
DELETE FROM emp WHERE ename IN (NULL, 'KING', 'FORD');
Furthermore, expressions of the form
value NOT IN set
yield FALSE if the set contains a null. For example, instead of deleting rows in which the ename column is not null and not 'KING', the following statement deletes no rows:
DELETE FROM emp WHERE ename NOT IN (NULL, 'KING');
Double vertical bars (||) serve as the concatenation operator, which appends one string to another, as the following example shows:
'suit' || 'case' = 'suitcase'
If both operands have datatype CHAR, the concatenation operator returns a CHAR value. Otherwise, it returns a VARCHAR2 value.
PL/SQL lets you compare variables and constants in both SQL and procedural statements. These comparisons, called Boolean expressions, consist of simple or complex expressions separated by relational operators. Often, Boolean expressions are connected by the logical operators AND, OR, and NOT. A Boolean expression always yields TRUE, FALSE, or NULL.
In a SQL statement, Boolean expressions let you specify the rows in a table that are affected by the statement. In a procedural statement, Boolean expressions are the basis for conditional control. There are three kinds of Boolean expressions: arithmetic, character, and date.
You can use the relational operators to compare numbers for equality or inequality. Comparisons are quantitative; that is, one number is greater than another if it represents a larger quantity. For example, given the assignments
number1 := 75; number2 := 70;
the following expression yields TRUE:
number1 > number2
Likewise, you can compare character values for equality or inequality. Comparisons are based on the collating sequence used for the database character set. A collating sequence is an internal ordering of the character set, in which a range of numeric codes represents the individual characters. One character value is greater than another if its internal numeric value is larger. For example, given the assignments
string1 := 'Kathy'; string2 := 'Kathleen';
the following expression yields TRUE:
string1 > string2
However, there are semantic differences between the CHAR and VARCHAR2 base types that come into play when you compare character values. For more information, refer to Appendix C.
You can also compare dates. Comparisons are chronological; that is, one date is greater than another if it is more recent. For example, given the assignments
date1 := '01-JAN-91'; date2 := '31-DEC-90';
the following expression yields TRUE:
date1 > date2
In general, do not compare real numbers for exact equality or inequality. Real numbers are stored as approximate values. So, for example, the following IF condition might not yield TRUE:
count := 1; IF count = 1.0 THEN ...
It is a good idea to use parentheses when doing comparisons. For example, the following expression is illegal because 100 < tax yields TRUE or FALSE, which cannot be compared with the number 500:
100 < tax < 500 -- illegal
The debugged version follows:
(100 < tax) AND (tax < 500)
A Boolean variable is itself either true or false. So, comparisons with the Boolean values TRUE and FALSE are redundant. For example, assuming the variable done is of type BOOLEAN, the WHILE statement
WHILE NOT (done = TRUE) LOOP ... END LOOP;
can be simplified as follows:
WHILE NOT done LOOP ... END LOOP;
When working with nulls, you can avoid some common mistakes by keeping in mind the following rules:
NULL
NOT to a null yields NULL
NULL, its associated sequence of statements is not executed
In the example below, you might expect the sequence of statements to execute because x and y seem unequal. But, nulls are indeterminate. Whether or not x is equal to y is unknown. Therefore, the IF condition yields NULL and the sequence of statements is bypassed.
x := 5; y := NULL; ... IF x != y THEN -- yields NULL, not TRUE sequence_of_statements; -- not executed END IF;
In the next example, you might expect the sequence of statements to execute because a and b seem equal. But, again, that is unknown, so the IF condition yields NULL and the sequence of statements is bypassed.
a := NULL; b := NULL; ... IF a = b THEN -- yields NULL, not TRUE sequence_of_statements; -- not executed END IF;
Recall that applying the logical operator NOT to a null yields NULL. Thus, the following two statements are not always equivalent:
IF x > y THEN | IF NOT x > y THEN high := x; | high := y; ELSE | ELSE high := y; | high := x; END IF; | END IF;
The sequence of statements in the ELSE clause is executed when the IF condition yields FALSE or NULL. So, if either or both x and y are null, the first IF statement assigns the value of y to high, but the second IF statement assigns the value of x to high. If neither x nor y is null, both IF statements assign the same value to high.
PL/SQL treats any zero-length string like a null. This includes values returned by character functions and Boolean expressions. For example, the following statements assign nulls to the target variables:
null_string := TO_VARCHAR2(''); zip_code := SUBSTR(address, 25, 0); valid := (name != '');
So, use the IS NULL operator to test for null strings, as follows:
IF my_string IS NULL THEN ...
The concatenation operator ignores null operands. For example, the expression
'apple' || NULL || NULL || 'sauce' = 'applesauce'
If a null argument is passed to a built-in function, a null is returned except in the following cases.
The function DECODE compares its first argument to one or more search expressions, which are paired with result expressions. Any search or result expression can be null. If a search is successful, the corresponding result is returned. In the following example, if the column rating is null, DECODE returns the value 1000:
SELECT DECODE(rating, NULL, 1000, 'C', 2000, 'B', 4000, 'A', 5000) INTO credit_limit FROM accts WHERE acctno = my_acctno;
The function NVL returns the value of its second argument if its first argument is null. In the example below, if hire_date is null, NVL returns the value of SYSDATE. Otherwise, NVL returns the value of hire_date:
start_date := NVL(hire_date, SYSDATE);
The function REPLACE returns the value of its first argument if its second argument is null, whether the optional third argument is present or not. For instance, after the assignment
new_string := REPLACE(old_string, NULL, my_string);
the values of old_string and new_string are the same.
If its third argument is null, REPLACE returns its first argument with every occurrence of its second argument removed. For example, after the assignments
syllabified_name := 'Gold-i-locks'; name := REPLACE(syllabified_name, '-', NULL);
the value of name is 'goldilocks'
If its second and third arguments are null, REPLACE simply returns its first argument.
PL/SQL provides many powerful functions to help you manipulate data. These built-in functions fall into the following categories:
Table 2-4 shows the functions in each category. For descriptions of the error-reporting functions, see Chapter 11. For descriptions of the other functions, see Oracle8 SQL Reference.
You can use all the functions in SQL statements except the error-reporting functions SQLCODE and SQLERRM. Also, you can use all the functions in procedural statements except the miscellaneous functions DECODE, DUMP, and VSIZE.
The SQL group functions AVG, MIN, MAX, COUNT, SUM, STDDEV, and VARIANCE are not built into PL/SQL. Nevertheless, you can use them in SQL statements (but not in procedural statements).
