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Thursday, February 16, 2012

What is SQL Having Clause?

Having clause is used to filter data based on the group functions. This is similar to WHERE condition but is used with group functions. Group functions cannot be used in WHERE Clause but can be used in HAVING clause.
For Example: If you want to select the department that has total salary paid for its employees more than 25000, the sql query would be like;
SELECT dept, SUM (salary)
FROM employee
GROUP BY dept
HAVING SUM (salary) > 25000

The output would be like:
dept salary
------------- -------------
Electronics 55000
Aeronautics 35000
InfoTech 30000
When WHERE, GROUP BY and HAVING clauses are used together in a SELECT statement, the WHERE clause is processed first, then the rows that are returned after the WHERE clause is executed are grouped based on the GROUP BY clause. Finally, any conditions on the group functions in the HAVING clause are applied to the grouped rows before the final output is displayed.

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