Friday, 22 June 2012

4. HAVING clause in SQL

The HAVING clause allows you to specify conditions on the rows for each group - in other words, which rows should be selected will be based on the conditions you specify. The HAVING clause should follow the GROUP BY clause if you are going to use it.

HAVING clause syntax:

SELECT column1, 

FROM "list-of-tables"

GROUP BY "column-list"

HAVING "condition";

HAVING can best be described by example. Let's say you have an employee table containing the employee's name, department, salary, and age. If you would like to select the average salary for each employee in each department, you could enter:

SELECT dept, avg(salary)

FROM employee

GROUP BY dept;

But, let's say that you want to ONLY calculate & display the average if their salary is over 20000:

SELECT dept, avg(salary)

FROM employee


HAVING avg(salary) > 20000;

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