Course Notes:
Chapter 9:
SQL is a standard computer language for accessing and manipulating databases.
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What is SQL?
* SQL stands for Structured Query Language
* SQL allows you to access a database
* SQL is an ANSI standard computer language
* SQL can execute queries against a database
* SQL can retrieve data from a database
* SQL can insert new records in a database
* SQL can delete records from a database
* SQL can update records in a database
* SQL is easy to learn
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SQL is a Standard - BUT....
SQL is an ANSI (American National Standards Institute) standard computer language for accessing and manipulating database systems. SQL statements are used to retrieve and update data in a database. SQL works with database programs like MS Access, DB2, Informix, MS SQL Server, Oracle, Sybase, etc.
Unfortunately, there are many different versions of the SQL language, but to be in compliance with the ANSI standard, they must support the same major keywords in a similar manner (such as SELECT, UPDATE, DELETE, INSERT, WHERE, and others).
Note: Most of the SQL database programs also have their own proprietary extensions in addition to the SQL standard!
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SQL Create Database and Table
Create a Database
To create a database:
CREATE DATABASE database_name
Create a Table
To create a table in a database:
CREATE TABLE table_name
(
column_name1 data_type, column_name2 data_type,
)
Example
This example demonstrates how you can create a table named "Person", with four columns. The column names will be "LastName", "FirstName", "Address", and "Age":
CREATE TABLE Person
(
LastName varchar,
FirstName varchar,
Address varchar,
Age int
)
This example demonstrates how you can specify a maximum length for some columns:
CREATE TABLE Person
(
LastName varchar(30),
FirstName varchar,
Address varchar,
Age int(3)
)
The data type specifies what type of data the column can hold. The table below contains the most common data types in SQL:
Data Type Description
integer(size)
int(size)
smallint(size)
tinyint(size) Hold integers only. The maximum number of digits are specified in parenthesis.
decimal(size,d)
numeric(size,d) Hold numbers with fractions. The maximum number of digits are specified in "size". The maximum number of digits to the right of the decimal is specified in "d".
char(size) Holds a fixed length string (can contain letters, numbers, and special characters). The fixed size is specified in parenthesis.
varchar(size) Holds a variable length string (can contain letters, numbers, and special characters). The maximum size is specified in parenthesis.
date(yyyymmdd) Holds a date
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SQL Database Tables
A database most often contains one or more tables. Each table is identified by a name (e.g. "Customers" or "Orders"). Tables contain records (rows) with data.
Below is an example of a table called "Persons":
LastName FirstName Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes
Pettersen Kari Storgt 20 Stavanger
The table above contains three records (one for each person) and four columns (LastName, FirstName, Address, and City).
SQL Queries
With SQL, we can query a database and have a result set returned.
SELECT field FROM table
A query like this:
SELECT LastName FROM Persons
Gives a result set like this:
LastName
Hansen
Svendson
Pettersen
Note: Some database systems require a semicolon at the end of the SQL statement. We don't use the semicolon in our tutorials.
SQL Data Manipulation Language (DML)
SQL (Structured Query Language) is a syntax for executing queries. But the SQL language also includes a syntax to update, insert, and delete records.
These query and update commands together form the Data Manipulation Language (DML) part of SQL:
* SELECT - extracts data from a database table
* UPDATE - updates data in a database table
* DELETE - deletes data from a database table
* INSERT INTO - inserts new data into a database table
SQL Data Definition Language (DDL)
The Data Definition Language (DDL) part of SQL permits database tables to be created or deleted. We can also define indexes (keys), specify links between tables, and impose constraints between database tables.
The most important DDL statements in SQL are:
* CREATE TABLE - creates a new database table
* ALTER TABLE - alters (changes) a database table
* DROP TABLE - deletes a database table
* CREATE INDEX - creates an index (search key)
* DROP INDEX - deletes an index
The SELECT Statement
The SELECT statement is used to select data from a table. The tabular result is stored in a result table (called the result-set).
Syntax
SELECT column_name(s) FROM table_name
Select Some Columns
To select the columns named "LastName" and "FirstName", use a SELECT statement like this:
SELECT LastName, FirstName FROM Persons
"Persons" table
LastName FirstName Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes
Pettersen Kari Storgt 20 Stavanger
Result
LastName FirstName
Hansen Ola
Svendson Tove
Pettersen Kari
------------------------------------------------------------------------
Select All Columns
To select all columns from the "Persons" table, use a * symbol instead of column names, like this:
SELECT * FROM Persons
Result
LastName FirstName Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes
Pettersen Kari Storgt 20 Stavanger
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The Result Set
The result from a SQL query is stored in a result-set. Most database software systems allow navigation of the result set with programming functions, like: Move-To-First-Record, Get-Record-Content, Move-To-Next-Record, etc.
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Semicolon after SQL Statements?
Semicolon is the standard way to separate each SQL statement in database systems that allow more than one SQL statement to be executed in the same call to the server.
Some SQL tutorials ends each SQL statement with a semicolon. Is this necessary? We are using MS Access and SQL Server 2000 and we do not have to put a semicolon after each SQL statement, but some database programs force you to use it.
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The SELECT DISTINCT Statement
The DISTINCT keyword is used to return only distinct (different) values.
The SELECT statement returns information from table columns. But what if we only want to select distinct elements?
With SQL, all we need to do is to add a DISTINCT keyword to the SELECT statement:
Syntax
SELECT DISTINCT column_name(s)
FROM table_name
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Using the DISTINCT keyword
To select ALL values from the column named "Company" we use a SELECT statement like this:
SELECT Company FROM Orders
"Orders" table
Company OrderNumber
Sega 3412
W3Schools 2312
Trio 4678
W3Schools 6798
Result
Company
Sega
W3Schools
Trio
W3Schools
Note that "W3Schools" is listed twice in the result-set.
To select only DIFFERENT values from the column named "Company" we use a SELECT DISTINCT statement like this:
SELECT DISTINCT Company FROM Orders
Result:
Company
Sega
W3Schools
Trio
Now "W3Schools" is listed only once in the result-set.
The WHERE Clause
To conditionally select data from a table, a WHERE clause can be added to the SELECT statement.
Syntax
SELECT column FROM table
WHERE column operator value
With the WHERE clause, the following operators can be used:
Operator Description
= Equal
<> Not equal
> Greater than
< Less than
>= Greater than or equal
<= Less than or equal
BETWEEN Between an inclusive range
LIKE
Search for a pattern
Note: In some versions of SQL the <> operator may be written as !=
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Using the WHERE Clause
To select only the persons living in the city "Sandnes", we add a WHERE clause to the SELECT statement:
SELECT * FROM Persons
WHERE City='Sandnes'
"Persons" table
LastName FirstName Address City Year
Hansen Ola Timoteivn 10 Sandnes 1951
Svendson Tove Borgvn 23 Sandnes 1978
Svendson Stale Kaivn 18 Sandnes 1980
Pettersen Kari Storgt 20 Stavanger 1960
Result:
LastName FirstName Address City Year
Hansen Ola Timoteivn 10 Sandnes 1951
Svendson Tove Borgvn 23 Sandnes 1978
Svendson Stale Kaivn 18 Sandnes 1980
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Using Quotes
Note that we have used single quotes around the conditional values in the examples.
SQL uses single quotes around text values (most database systems will also accept double quotes). Numeric values should not be enclosed in quotes.
For text values:
This is correct:
SELECT * FROM Persons WHERE FirstName='Tove'
This is wrong:
SELECT * FROM Persons WHERE FirstName=Tove
For numeric values:
This is correct:
SELECT * FROM Persons WHERE Year>1965
This is wrong:
SELECT * FROM Persons WHERE Year>'1965'
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The LIKE Condition
The LIKE condition is used to specify a search for a pattern in a column.
Syntax
SELECT column FROM table
WHERE column LIKE pattern
A "%" sign can be used to define wildcards (missing letters in the pattern) both before and after the pattern.
Using LIKE
The following SQL statement will return persons with first names that start with an 'O':
SELECT * FROM Persons
WHERE FirstName LIKE 'O%'
The following SQL statement will return persons with first names that end with an 'a':
SELECT * FROM Persons
WHERE FirstName LIKE '%a'
The following SQL statement will return persons with first names that contain the pattern 'la':
SELECT * FROM Persons
WHERE FirstName LIKE '%la%'
The INSERT INTO Statement
The INSERT INTO statement is used to insert new rows into a table.
Syntax
INSERT INTO table_name
VALUES (value1, value2,....)
You can also specify the columns for which you want to insert data:
INSERT INTO table_name (column1, column2,...)
VALUES (value1, value2,....)
Insert a New Row
This "Persons" table:
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
And this SQL statement:
INSERT INTO Persons
VALUES ('Hetland', 'Camilla', 'Hagabakka 24', 'Sandnes')
Will give this result:
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
Hetland Camilla Hagabakka 24 Sandnes
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Insert Data in Specified Columns
This "Persons" table:
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
Hetland Camilla Hagabakka 24 Sandnes
And This SQL statement:
INSERT INTO Persons (LastName, Address)
VALUES ('Rasmussen', 'Storgt 67')
Will give this result:
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
Hetland Camilla Hagabakka 24 Sandnes
Rasmussen Storgt 67
The Update Statement
The UPDATE statement is used to modify the data in a table.
Syntax
UPDATE table_name
SET column_name = new_value
WHERE column_name = some_value
------------------------------------------------------------------------
Person:
LastName FirstName Address City
Nilsen Fred Kirkegt 56 Stavanger
Rasmussen Storgt 67
------------------------------------------------------------------------
Update one Column in a Row
We want to add a first name to the person with a last name of "Rasmussen":
UPDATE Person SET FirstName = 'Nina'
WHERE LastName = 'Rasmussen'
Result:
LastName FirstName Address City
Nilsen Fred Kirkegt 56 Stavanger
Rasmussen Nina Storgt 67
------------------------------------------------------------------------
Update several Columns in a Row
We want to change the address and add the name of the city:
UPDATE Person
SET Address = 'Stien 12', City = ' Stavanger'
WHERE LastName = 'Rasmussen'
Result:
LastName FirstName Address City
Nilsen Fred Kirkegt 56 Stavanger
Rasmussen Nina Stien 12 Stavanger
The Delete Statement
The DELETE statement is used to delete rows in a table.
Syntax
DELETE FROM table_name
WHERE column_name = some_value
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Person:
LastName FirstName Address City
Nilsen Fred Kirkegt 56 Stavanger
Rasmussen Nina Stien 12 Stavanger
------------------------------------------------------------------------
Delete a Row
"Nina Rasmussen" is going to be deleted:
DELETE FROM Person WHERE LastName = 'Rasmussen'
Result
LastName FirstName Address City
Nilsen Fred Kirkegt 56 Stavanger
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Delete All Rows
It is possible to delete all rows in a table without deleting the table. This means that the table structure, attributes, and indexes will be intact:
DELETE FROM table_name
or
DELETE * FROM table_name
SQL Order By
------------------------------------------------------------------------
The ORDER BY keywords are used to sort-order the result.
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Sort the Rows
The ORDER BY clause is used to sort the rows.
Orders:
Company OrderNumber
Sega 3412
ABC Shop 5678
W3Schools 2312
W3Schools 6798
Example
To display the companies in alphabetical order:
SELECT Company, OrderNumber FROM Orders
ORDER BY Company
Result:
Company OrderNumber
ABC Shop 5678
Sega 3412
W3Schools 6798
W3Schools 2312
Example
To display the companies in alphabetical order AND the ordernumbers in numerical order:
SELECT Company, OrderNumber FROM Orders
ORDER BY Company, OrderNumber
Result:
Company OrderNumber
ABC Shop 5678
Sega 3412
W3Schools 2312
W3Schools 6798
Example
To display the companies in reverse alphabetical order:
SELECT Company, OrderNumber FROM Orders
ORDER BY Company DESC
Result:
Company OrderNumber
W3Schools 6798
W3Schools 2312
Sega 3412
ABC Shop 5678
Example
To display the companies in reverse alphabetical order AND the ordernumbers in numerical order:
SELECT Company, OrderNumber FROM Orders
ORDER BY Company DESC, OrderNumber ASC
Result:
Company OrderNumber
W3Schools 2312
W3Schools 6798
Sega 3412
ABC Shop 5678
SQL And & Or
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AND & OR
AND and OR join two or more conditions in a WHERE clause.
The AND operator displays a row if ALL conditions listed are true. The OR operator displays a row if ANY of the conditions listed are true.
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Original Table (used in the examples)
LastName FirstName Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes
Svendson Stephen Kaivn 18 Sandnes
------------------------------------------------------------------------
Example
Use AND to display each person with the first name equal to "Tove", and the last name equal to "Svendson":
SELECT * FROM Persons
WHERE FirstName='Tove'
AND LastName='Svendson'
Result:
LastName FirstName Address City
Svendson Tove Borgvn 23 Sandnes
Example
Use OR to display each person with the first name equal to "Tove", or the last name equal to "Svendson":
SELECT * FROM Persons
WHERE firstname='Tove'
OR lastname='Svendson'
Result:
LastName FirstName Address City
Svendson Tove Borgvn 23 Sandnes
Svendson Stephen Kaivn 18 Sandnes
Example
You can also combine AND and OR (use parentheses to form complex expressions):
SELECT * FROM Persons WHERE
(FirstName='Tove' OR FirstName='Stephen')
AND LastName='Svendson'
Result:
LastName FirstName Address City
Svendson Tove Borgvn 23 Sandnes
Svendson Stephen Kaivn 18 Sandnes
SQL Between...And
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BETWEEN ... AND
The BETWEEN ... AND operator selects a range of data between two values. These values can be numbers, text, or dates.
SELECT column_name FROM table_name
WHERE column_name
BETWEEN value1 AND value2
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Original Table (used in the examples)
LastName FirstName Address City
Hansen Ola Timoteivn 10 Sandnes
Nordmann Anna Neset 18 Sandnes
Pettersen Kari Storgt 20 Stavanger
Svendson Tove Borgvn 23 Sandnes
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Example 1
To display the persons alphabetically between (and including) "Hansen" and exclusive "Pettersen", use the following SQL:
SELECT * FROM Persons WHERE LastName
BETWEEN 'Hansen' AND 'Pettersen'
Result:
LastName FirstName Address City
Hansen Ola Timoteivn 10 Sandnes
Nordmann Anna Neset 18 Sandnes
IMPORTANT! The BETWEEN...AND operator is treated differently in different databases. With some databases a person with the LastName of "Hansen" or "Pettersen" will not be listed (BETWEEN..AND only selects fields that are between and excluding the test values). With some databases a person with the last name of "Hansen" or "Pettersen" will be listed (BETWEEN..AND selects fields that are between and including the test values). With other databases a person with the last name of "Hansen" will be listed, but "Pettersen" will not be listed (BETWEEN..AND selects fields between the test values, including the first test value and excluding the last test value). Therefore: Check how your database treats the BETWEEN....AND operator!
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Example 2
To display the persons outside the range used in the previous example, use the NOT operator:
SELECT * FROM Persons WHERE LastName
NOT BETWEEN 'Hansen' AND 'Pettersen'
Result:
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
Svendson Tove Borgvn 23 Sandnes
SQL Aliases
------------------------------------------------------------------------
With SQL, aliases can be used for column names and table names.
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Column Name Alias
The syntax is:
SELECT column AS column_alias FROM table
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Table Name Alias
The syntax is:
SELECT column FROM table AS table_alias
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Example: Using a Column Alias
This table (Persons):
LastName FirstName Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes
Pettersen Kari Storgt 20 Stavanger
And this SQL:
SELECT LastName AS Family, FirstName AS Name
FROM Persons
Returns this result:
Family Name
Hansen Ola
Svendson Tove
Pettersen Kari
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Example: Using a Table Alias
This table (Persons):
LastName FirstName Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes
Pettersen Kari Storgt 20 Stavanger
And this SQL:
SELECT LastName, FirstName
FROM Persons AS Employees
Returns this result:
Table Employees:
LastName FirstName
Hansen Ola
Svendson Tove
Pettersen Kari
SQL Join
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Joins and Keys
Sometimes we have to select data from two tables to make our result complete. We have to perform a join.
Tables in a database can be related to each other with keys. A primary key is a column with a unique value for each row. The purpose is to bind data together, across tables, without repeating all of the data in every table.
In the "Employees" table below, the "Employee_ID" column is the primary key, meaning that no two rows can have the same Employee_ID. The Employee_ID distinguishes two persons even if they have the same name.
When you look at the example tables below, notice that:
* The "Employee_ID" column is the primary key of the "Employees" table
* The "Prod_ID" column is the primary key of the "Orders" table
* The "Employee_ID" column in the "Orders" table is used to refer to the persons in the "Employees" table without using their names
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Employees:
Employee_ID Name
01 Hansen, Ola
02 Svendson, Tove
03 Svendson, Stephen
04 Pettersen, Kari
Orders:
Prod_ID Product Employee_ID
234 Printer 01
657 Table 03
865 Chair 03
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Referring to Two Tables
We can select data from two tables by referring to two tables, like this:
Example
Who has ordered a product, and what did they order?
SELECT Employees.Name, Orders.Product
FROM Employees, Orders
WHERE Employees.Employee_ID=Orders.Employee_ID
Result
Name Product
Hansen, Ola Printer
Svendson, Stephen Table
Svendson, Stephen Chair
Example
Who ordered a printer?
SELECT Employees.Name
FROM Employees, Orders
WHERE Employees.Employee_ID=Orders.Employee_ID
AND Orders.Product='Printer'
Result
Name
Hansen, Ola
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Using Joins
OR we can select data from two tables with the JOIN keyword, like this:
Example INNER JOIN
Syntax
SELECT field1, field2, field3
FROM first_table
INNER JOIN second_table
ON first_table.keyfield = second_table.foreign_keyfield
Who has ordered a product, and what did they order?
SELECT Employees.Name, Orders.Product
FROM Employees
INNER JOIN Orders
ON Employees.Employee_ID=Orders.Employee_ID
The INNER JOIN returns all rows from both tables where there is a match. If there are rows in Employees that do not have matches in Orders, those rows will not be listed.
Result
Name Product
Hansen, Ola Printer
Svendson, Stephen Table
Svendson, Stephen Chair
Example LEFT JOIN
Syntax
SELECT field1, field2, field3
FROM first_table
LEFT JOIN second_table
ON first_table.keyfield = second_table.foreign_keyfield
List all employees, and their orders - if any.
SELECT Employees.Name, Orders.Product
FROM Employees
LEFT JOIN Orders
ON Employees.Employee_ID=Orders.Employee_ID
The LEFT JOIN returns all the rows from the first table (Employees), even if there are no matches in the second table (Orders). If there are rows in Employees that do not have matches in Orders, those rows also will be listed.
Result
Name Product
Hansen, Ola Printer
Svendson, Tove
Svendson, Stephen Table
Svendson, Stephen Chair
Pettersen, Kari
Example RIGHT JOIN
Syntax
SELECT field1, field2, field3
FROM first_table
RIGHT JOIN second_table
ON first_table.keyfield = second_table.foreign_keyfield
List all orders, and who has ordered - if any.
SELECT Employees.Name, Orders.Product
FROM Employees
RIGHT JOIN Orders
ON Employees.Employee_ID=Orders.Employee_ID
The RIGHT JOIN returns all the rows from the second table (Orders), even if there are no matches in the first table (Employees). If there had been any rows in Orders that did not have matches in Employees, those rows also would have been listed.
Result
Name Product
Hansen, Ola Printer
Svendson, Stephen Table
Svendson, Stephen Chair
Example
Who ordered a printer?
SELECT Employees.Name
FROM Employees
INNER JOIN Orders
ON Employees.Employee_ID=Orders.Employee_ID
WHERE Orders.Product = 'Printer'
Result
Name
Hansen, Ola
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