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SQL Where Clause




The WHERE clause is used to specify a selection criteria.


The WHERE Clause 
To conditionally select data from a table, a WHERE clause can be added to the SELECT
statement with the following syntax:



SELECT column FROM table WHERE column condition value



With the WHERE clause, these conditions can be used:


Operator
Condition


=
Equal


<>
Not equal


>
Greater than


<
Less than


>=
Greater than or equal


<=
Less than or equal


LIKE
Explained below



Note: In some versions of SQL the not equal operator <> can be written as
!=


Example: Select Persons from a City
To select only the
people that live in Sandnes, add a WHERE clause to the SELECT statement like
this: 



SELECT * FROM Persons WHERE City='Sandnes'




The "Persons" Table:


LastName
FirstName
Address
City
Year


Hansen
Ola
Timoteivn 10
Sandnes
1951


Svendson
Tove
Borgvn 23
Sandnes
1978


Svendson
Ståle
Kaivn 18
Sandnes
1980


Pettersen
Kari
Storgt 20
Stavanger
1960


The Result:


LastName
FirstName
Address
City
Year


Hansen
Ola
Timoteivn 10
Sandnes
1951


Svendson
Tove
Borgvn 23
Sandnes
1978


Svendson
Ståle
Kaivn 18
Sandnes
1980





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 not correct:
SELECT * FROM Persons WHERE FirstName=Tove


For Numeric values:


This is correct:
SELECT * FROM Persons WHERE Year>1965This is not correct:
SELECT * FROM Persons WHERE Year>'1965'





The LIKE Condition


The LIKE condition is used to specify a search for a pattern in a column.


The syntax is like this:




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.

Example: Select Persons with a Name Pattern 

This SQL statement will return persons with a first name that start with an 'O'.



SELECT * FROM Persons WHERE FirstName LIKE 'O%'


This SQL statement will return persons with a first name that end with an 'a'.



SELECT * FROM Persons WHERE FirstName LIKE '%a'



This SQL statement will return persons with a first name that contains the
pattern 'la'.



SELECT * FROM Persons WHERE FirstName LIKE '%la%'



All the examples above will return the following result:


LastName
FirstName
Address
City
Year


Hansen
Ola
Timoteivn 10
Sandnes
1951







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