The SQL GROUP BY Clause is used along with the group functions to retrieve data grouped according to one or more columns.
For Example: If you want to know the total amount of salary spent on each department, the query would be:
SELECT dept, SUM (salary)
GROUP BY dept;
The output would be like:
NOTE: The group by clause should contain all the columns in the select list expect those used along with the group functions.
SELECT location, dept, SUM (salary)
GROUP BY location, dept;