What is SQL GROUP BY Clause

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)
FROM employee
GROUP BY dept;

The output would be like:

dept	salary
----------------	--------------
Electrical	25000
Electronics	55000
Aeronautics	35000
InfoTech	30000

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)
FROM employee
GROUP BY location, dept;

The output would be like:

location	dept	salary
-------------	---------------	-----------
Bangalore	Electrical	25000
Bangalore	Electronics	55000
Mysore	Aeronautics	35000
Mangalore	InfoTech	30000