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Saturday, February 28, 2009

SQL HAVING

The SQL HAVING clause is used to restrict conditionally the output of a SQL statement, by a SQL aggregate function used in your SELECT list of columns.

You can't specify criteria in a SQL WHERE clause against a column in the SELECT list for which SQL aggregate function is used. For example the following SQL statement will generate an error:


SELECT Employee, SUM (Hours)
FROM EmployeeHours
WHERE SUM (Hours) > 24
GROUP BY Employee

The SQL HAVING clause is used to do exactly this, to specify a condition for an aggregate function which is used in your query:


SELECT Employee, SUM (Hours)
FROM EmployeeHours
GROUP BY Employee
HAVING SUM (Hours) > 24

The above SQL statement will select all employees and the sum of their respective hours, as long as this sum is greater than 24. The result of the SQL HAVING clause can be seen below:

Employee Hours
John Smith 25
Tina Crown 27
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