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:
SELECT * FROM Persons In this tutorial we will teach you all about the different SQL statements.
Keep in Mind That...
- SQL is not case sensitive
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
- 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
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
SELECT * FROM table_name 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:
SELECT * FROM Persons 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
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:
City Sandnes Stavanger
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 !=
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
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
THE MOST IMPORTANT PART
The following table lists the most important built-in date functions in MySQL:
Function Description
NOW() Returns the current date and time
CURDATE() Returns the current date
CURTIME() Returns the current time
DATE() Extracts the date part of a date or date/time expression
EXTRACT() Returns a single part of a date/time
DATE_ADD() Adds a specified time interval to a date
DATE_SUB() Subtracts a specified time interval from a date
DATEDIFF() Returns the number of days between two dates
DATE_FORMAT() Displays date/time data in different formats
SQL Server Date Functions
The following table lists the most important built-in date functions in SQL Server:
Function Description
GETDATE() Returns the current date and time
DATEPART() Returns a single part of a date/time
DATEADD() Adds or subtracts a specified time interval from a date
DATEDIFF() Returns the time between two dates
CONVERT() Displays date/time data in different formats
SQL Date Data Types
MySQL comes with the following data types for storing a date or a date/time value in the database:
DATE - format YYYY-MM-DD
DATETIME - format: YYYY-MM-DD HH:MM:SS
TIMESTAMP - format: YYYY-MM-DD HH:MM:SS
YEAR - format YYYY or YY
SQL Server comes with the following data types for storing a date or a date/time value in the database:
DATE - format YYYY-MM-DD
DATETIME - format: YYYY-MM-DD HH:MM:SS
SMALLDATETIME - format: YYYY-MM-DD HH:MM:SS
TIMESTAMP - format: a unique number
Note: The date types are chosen for a column when you create a new table in your database!
For an overview of all data types available, go to our complete Data Types reference.
SQL Working with Dates
Note You can compare two dates easily if there is no time component involved!
Assume we have the following "Orders" table:
OrderId ProductName OrderDate
1 Geitost 2008-11-11
2 Camembert Pierrot 2008-11-09
3 Mozzarella di Giovanni 2008-11-11
4 Mascarpone Fabioli 2008-10-29
Now we want to select the records with an OrderDate of "2008-11-11" from the table above.
We use the following SELECT statement:
SELECT * FROM Orders WHERE OrderDate='2008-11-11'
The result-set will look like this:
OrderId ProductName OrderDate
1 Geitost 2008-11-11
3 Mozzarella di Giovanni 2008-11-11
Now, assume that the "Orders" table looks like this (notice the time component in the "OrderDate" column):
OrderId ProductName OrderDate
1 Geitost 2008-11-11 13:23:44
2 Camembert Pierrot 2008-11-09 15:45:21
3 Mozzarella di Giovanni 2008-11-11 11:12:01
4 Mascarpone Fabioli 2008-10-29 14:56:59
If we use the same SELECT statement as above:
SELECT * FROM Orders WHERE OrderDate='2008-11-11'
we will get no result! This is because the query is looking only for dates with no time portion.
Tip: If you want to keep your queries simple and easy to maintain, do not allow time components in your dates!
-----------------
Conditional Statements
Very often when you write code, you want to perform different actions for different decisions.
You can use conditional statements in your code to do this.
In PHP we have the following conditional statements:
- if statement - use this statement to execute some code only if a specified condition is true
- if...else statement - use this statement to execute some code if a condition is true and another code if the condition is false
- if...elseif....else statement - use this statement to select one of several blocks of code to be executed
- switch statement - use this statement to select one of many blocks of code to be executed
Use the if statement to execute some code only if a specified condition is true.
Syntax
if (condition) code to be executed if condition is true; The following example will output "Have a nice weekend!" if the current day is Friday:
<html>
<body>
<?php
$d=date("D");
if ($d=="Fri") echo "Have a nice weekend!";
?>
</body>
</html> Notice that there is no ..else.. in this syntax. The code is executed only if the specified condition is true.
The if...else Statement
Use the if....else statement to execute some code if a condition is true and another code if a condition is false.
Syntax
if (condition)
code to be executed if condition is true;
else
code to be executed if condition is false; Example
The following example will output "Have a nice weekend!" if the current day is Friday, otherwise it will output "Have a nice day!":
<html>
<body>
<?php
$d=date("D");
if ($d=="Fri")
echo "Have a nice weekend!";
else
echo "Have a nice day!";
?>
</body>
</html> If more than one line should be executed if a condition is true/false, the lines should be enclosed within curly braces:
<html>
<body>
<?php
$d=date("D");
if ($d=="Fri")
{
echo "Hello!<br />";
echo "Have a nice weekend!";
echo "See you on Monday!";
}
?>
</body>
</html>
The if...elseif....else Statement
Use the if....elseif...else statement to select one of several blocks of code to be executed.
Syntax
if (condition)
code to be executed if condition is true;
elseif (condition)
code to be executed if condition is true;
else
code to be executed if condition is false; Example
The following example will output "Have a nice weekend!" if the current day is Friday, and "Have a nice Sunday!" if the current day is Sunday. Otherwise it will output "Have a nice day!":
<html>
<body>
<?php
$d=date("D");
if ($d=="Fri")
echo "Have a nice weekend!";
elseif ($d=="Sun")
echo "Have a nice Sunday!";
else
echo "Have a nice day!";
?>
</body>
</html>
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