Group functions are the built-in SQL functions that act on groups of rows and return one value for the entire group.
Rules of GroupBy clause
The name may be a Column reference. Such a Column is called a grouping Column: its values will be grouped for the final result.
Example:* The group by clause should contain all the columns in the select list expect those used along with the group functions.
Let us take the below query,
SELECT location, dept, SUM (salary)
FROM employee
GROUP BY location, dept;
In the above query, if the columns 'location' and 'dept' are not specified next to GroupBy clause, then it results an error.
* It is upto us to include aggregated columns next to GroupBy. Meaning, in the above mentioned example it is upto us to include column 'salary' next to the GroupBy clause.
* We cannot include a column next to GroupBy, if that particular column is not selected.