The WHERE clause is used to specify a selection criterion.
To conditionally select data from a table, a WHERE clause can be added to the SELECT statement.
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 |
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 !=
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 |
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' |
The LIKE condition is used to specify a search for a pattern in a column.
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.
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%' |
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