Introduction to SQL
SQL is a standard language for accessing and manipulating
databases. SQL, the Structured Query Language, is a mature, powerful, and versatile relational query
language. The history of SQL extends back to IBM research begun in 1970. The
next few sections discuss the history of SQL, its predecessors, and the various
SQL standards that have developed over the years.
What is SQL?
- SQL stands for Structured
Query Language
- SQL lets you access and
manipulate databases
- SQL is an ANSI (American
National Standards Institute) standard
What Can SQL do?
- SQL can execute queries
against a database
- SQL can retrieve data from
a database
- SQL can insert records in a
database
- SQL can update records in a
database
- SQL can delete records from
a database
- SQL can create new
databases
- SQL can create new tables
in a database
- SQL can create stored
procedures in a database
- SQL can create views in a
database
- SQL can set permissions on
tables, procedures, and views
SQL is a Standard - BUT....
Although SQL is an ANSI (American National Standards Institute) standard,
there are many different versions of the SQL language.
However, to be compliant with the ANSI standard, they all support at least
the major commands (such as SELECT, UPDATE, DELETE, INSERT, WHERE) in a similar
manner.
Note: Most of the SQL database programs also have their own
proprietary extensions in addition to the SQL standard!
Using SQL in Your Web Site
To build a web site that shows some data from a database, you will need the
following:
- An RDBMS database program
(i.e. MS Access, SQL Server, MySQL)
- A server-side scripting
language, like PHP or ASP
- SQL
- HTML / CSS
RDBMS
RDBMS stands for Relational Database Management System.
RDBMS is the basis for SQL, and for all modern database systems like MS SQL
Server, IBM DB2, Oracle, MySQL, and Microsoft Access.
The data in RDBMS is stored in database objects called tables.
A table is a collections of related data entries and it consists of columns
and rows.
SQL Syntax
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":
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
The table above contains three records (one for each person) and five
columns (P_Id, LastName, FirstName, Address, and City).
SQL Statements
Most of the actions you need to perform on a database are done with SQL
statements.
The following SQL statement will select all the records in the
"Persons" table:
In this tutorial we will teach you all about the different SQL statements.
Keep in Mind That...
- SQL is not case sensitive
Semicolon after SQL Statements?
Some database systems require a semicolon at the end of each SQL statement.
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.
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.
SQL DML and DDL
SQL can be divided into two parts: The Data Manipulation Language (DML) and
the Data Definition Language (DDL).
The query and update commands form the DML part of SQL:
- SELECT - extracts
data from a database
- UPDATE - updates
data in a database
- DELETE - deletes
data from a database
- INSERT INTO -
inserts new data into a database
The DDL part of SQL permits database tables to be created or deleted. It
also define indexes (keys), specify links between tables, and impose
constraints between tables. The most important DDL statements in SQL are:
- CREATE DATABASE -
creates a new database
- ALTER DATABASE -
modifies a database
- CREATE TABLE -
creates a new table
- ALTER TABLE -
modifies a table
- DROP TABLE - deletes
a table
- CREATE INDEX -
creates an index (search key)
- DROP INDEX - deletes
an index
SQL SELECT Statement
This chapter will explain the SELECT and the SELECT *
statements.
The SQL SELECT Statement
The SELECT statement is used to select data from a database.
The result is stored in a result table, called the result-set.
SQL SELECT Syntax
SELECT column_name(s)
FROM table_name
|
and
Note: SQL is not case
sensitive. SELECT is the same as select.
An SQL SELECT Example
The "Persons" table:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
Now we want to select the content of the columns named "LastName"
and "FirstName" from the table above.
We use the following SELECT statement:
SELECT LastName,FirstName FROM Persons
|
The result-set will look like this:
LastName
|
FirstName
|
Hansen
|
Ola
|
Svendson
|
Tove
|
Pettersen
|
Kari
|
SELECT * Example
Now we want to select all the columns from the "Persons" table.
We use the following SELECT statement:
Tip: The asterisk (*) is a quick way of selecting all columns!
The result-set will look like this:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
Navigation in a Result-set
Most database software systems allow navigation in the result-set with
programming functions, like: Move-To-First-Record, Get-Record-Content,
Move-To-Next-Record, etc.
The SQL SELECT DISTINCT Statement
In a table, some of the columns may contain duplicate values. This is not a
problem, however, sometimes you will want to list only the different (distinct)
values in a table.
The DISTINCT keyword can be used to return only distinct (different) values.
SQL SELECT DISTINCT Syntax
SELECT DISTINCT column_name(s)
FROM table_name
|
SELECT DISTINCT Example
The "Persons" table:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
Now we want to select only the distinct values from the column named
"City" from the table above.
We use the following SELECT statement:
SELECT DISTINCT City FROM Persons
|
The result-set will look like this:
SQL WHERE Clause
The WHERE clause is used to filter records.
The WHERE Clause
The WHERE clause is used to extract only those records that fulfill a
specified criterion.
SQL WHERE Syntax
SELECT column_name(s)
FROM table_name
WHERE column_name operator value
|
WHERE Clause Example
The "Persons" table:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
Now we want to select only the persons living in the city
"Sandnes" from the table above.
We use the following SELECT statement:
SELECT * FROM Persons
WHERE City='Sandnes'
|
The result-set will look like this:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
Quotes Around Text Fields
SQL uses single quotes around text values (most database systems will also
accept double quotes).
Although, 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'
|
Operators Allowed in the WHERE Clause
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
|
IN
|
If you know the exact value you want to return for at
least one of the columns
|
Note: In some versions of SQL the <> operator may be written as
!=
SQL AND & OR Operators
The AND & OR operators are used to filter records based on
more than one condition.
The AND & OR Operators
The AND operator displays a record if both the first condition and the
second condition is true.
The OR operator displays a record if either the first condition or the
second condition is true.
AND Operator Example
The "Persons" table:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
Now we want to select only the persons with the first name equal to
"Tove" AND the last name equal to "Svendson":
We use the following SELECT statement:
SELECT * FROM Persons
WHERE FirstName='Tove'
AND LastName='Svendson'
|
The result-set will look like this:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
OR Operator Example
Now we want to select only the persons with the first name equal to
"Tove" OR the first name equal to "Ola":
We use the following SELECT statement:
SELECT * FROM Persons
WHERE FirstName='Tove'
OR FirstName='Ola'
|
The result-set will look like this:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
Combining AND & OR
You can also combine AND and OR (use parenthesis to form complex
expressions).
Now we want to select only the persons with the last name equal to
"Svendson" AND the first name equal to "Tove" OR to
"Ola":
We use the following SELECT statement:
SELECT * FROM Persons WHERE
LastName='Svendson'
AND (FirstName='Tove' OR FirstName='Ola')
|
The result-set will look like this:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
SQL ORDER BY Keyword
The ORDER BY keyword is used to sort the result-set.
The ORDER BY Keyword
The ORDER BY keyword is used to sort the result-set by a specified column.
The ORDER BY keyword sort the records in ascending order by default.
If you want to sort the records in a descending order, you can use the DESC
keyword.
SQL ORDER BY Syntax
SELECT column_name(s)
FROM table_name
ORDER BY column_name(s) ASC|DESC
|
ORDER BY Example
The "Persons" table:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
4
|
Nilsen
|
Tom
|
Vingvn 23
|
Stavanger
|
Now we want to select all the persons from the table above, however, we want
to sort the persons by their last name.
We use the following SELECT statement:
SELECT * FROM Persons
ORDER BY LastName
|
The result-set will look like this:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
4
|
Nilsen
|
Tom
|
Vingvn 23
|
Stavanger
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
ORDER BY DESC Example
Now we want to select all the persons from the table above, however, we want
to sort the persons descending by their last name.
We use the following SELECT statement:
SELECT * FROM Persons
ORDER BY LastName DESC
|
The result-set will look like this:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
4
|
Nilsen
|
Tom
|
Vingvn 23
|
Stavanger
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
SQL INSERT INTO Statement
The INSERT INTO statement is used to insert new records in a
table.
The INSERT INTO Statement
The INSERT INTO statement is used to insert a new row in a table.
SQL INSERT INTO Syntax
It is possible to write the INSERT INTO statement in two forms.
The first form doesn't specify the column names where the data will be
inserted, only their values:
INSERT INTO table_name
VALUES (value1, value2, value3,...)
|
The second form specifies both the column names and the values to be
inserted:
INSERT INTO table_name (column1, column2, column3,...)
VALUES (value1, value2, value3,...)
|
SQL INSERT INTO Example
We have the following "Persons" table:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
Now we want to insert a new row in the "Persons" table.
We use the following SQL statement:
INSERT INTO Persons
VALUES (4,'Nilsen', 'Johan', 'Bakken 2', 'Stavanger')
|
The "Persons" table will now look like this:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
4
|
Nilsen
|
Johan
|
Bakken 2
|
Stavanger
|
Insert Data Only in Specified Columns
It is also possible to only add data in specific columns.
The following SQL statement will add a new row, but only add data in the
"P_Id", "LastName" and the "FirstName" columns:
INSERT INTO Persons (P_Id, LastName, FirstName)
VALUES (5, 'Tjessem', 'Jakob')
|
The "Persons" table will now look like this:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
4
|
Nilsen
|
Johan
|
Bakken 2
|
Stavanger
|
5
|
Tjessem
|
Jakob
|
|
|
SQL UPDATE Statement
The UPDATE statement is used to update records in a table.
The UPDATE Statement
The UPDATE statement is used to update existing records in a table.
SQL UPDATE Syntax
UPDATE table_name
SET column1=value, column2=value2,...
WHERE some_column=some_value
|
Note: Notice the WHERE clause in the UPDATE syntax. The WHERE clause
specifies which record or records that should be updated. If you omit the WHERE
clause, all records will be updated!
SQL UPDATE Example
The "Persons" table:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
4
|
Nilsen
|
Johan
|
Bakken 2
|
Stavanger
|
5
|
Tjessem
|
Jakob
|
|
|
Now we want to update the person "Tjessem, Jakob" in the
"Persons" table.
We use the following SQL statement:
UPDATE Persons
SET Address='Nissestien 67', City='Sandnes'
WHERE LastName='Tjessem' AND FirstName='Jakob'
|
The "Persons" table will now look like this:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
4
|
Nilsen
|
Johan
|
Bakken 2
|
Stavanger
|
5
|
Tjessem
|
Jakob
|
Nissestien 67
|
Sandnes
|
SQL UPDATE Warning
Be careful when updating records. If we had omitted the WHERE clause in the
example above, like this:
UPDATE Persons
SET Address='Nissestien 67', City='Sandnes'
|
The "Persons" table would have looked like this:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Nissestien 67
|
Sandnes
|
2
|
Svendson
|
Tove
|
Nissestien 67
|
Sandnes
|
3
|
Pettersen
|
Kari
|
Nissestien 67
|
Sandnes
|
4
|
Nilsen
|
Johan
|
Nissestien 67
|
Sandnes
|
5
|
Tjessem
|
Jakob
|
Nissestien 67
|
Sandnes
|
SQL DELETE Statement
The DELETE statement is used to delete records in a table.
The DELETE Statement
The DELETE statement is used to delete rows in a table.
SQL DELETE Syntax
DELETE FROM table_name
WHERE some_column=some_value
|
Note: Notice the WHERE clause in the DELETE syntax. The WHERE clause
specifies which record or records that should be deleted. If you omit the WHERE
clause, all records will be deleted!
SQL DELETE Example
The "Persons" table:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
4
|
Nilsen
|
Johan
|
Bakken 2
|
Stavanger
|
5
|
Tjessem
|
Jakob
|
Nissestien 67
|
Sandnes
|
Now we want to delete the person "Tjessem, Jakob" in the
"Persons" table.
We use the following SQL statement:
DELETE FROM Persons
WHERE LastName='Tjessem' AND FirstName='Jakob'
|
The "Persons" table will now look like this:
P_Id
|
LastName
|
FirstName
|
Address
|
City
|
1
|
Hansen
|
Ola
|
Timoteivn 10
|
Sandnes
|
2
|
Svendson
|
Tove
|
Borgvn 23
|
Sandnes
|
3
|
Pettersen
|
Kari
|
Storgt 20
|
Stavanger
|
4
|
Nilsen
|
Johan
|
Bakken 2
|
Stavanger
|
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
|
Note: Be very careful when deleting records. You cannot undo this
statement!
Thank you ..:-)
