Average

The AVG() aggregate function will return the average of values selected.

SELECT AVG(Salary) FROM Employees

Aggregate functions can also be combined with the where clause.

SELECT AVG(Salary) FROM Employees where DepartmentId = 1

Aggregate functions can also be combined with group by clause.

If employee is categorized with multiple department and we want to find avg salary for every department then we can use following query.

SELECT AVG(Salary) FROM Employees GROUP BY DepartmentId

Minimum

The MIN() aggregate function will return the minimum of values selected.

SELECT MIN(Salary) FROM Employees

Maximum

The MAX() aggregate function will return the maximum of values selected.

SELECT MAX(Salary) FROM Employees

Count

The COUNT() aggregate function will return the count of values selected.

SELECT Count(*) FROM Employees

It can also be combined with where conditions to get the count of rows that satisfy specific conditions.

SELECT Count(*) FROM Employees where ManagerId IS NOT NULL

Specific columns can also be specified to get the number of values in the column. Note that NULL values are not counted.

Select Count(ManagerId) from Employees

Count can also be combined with the distinct keyword for a distinct count.

Select Count(DISTINCT DepartmentId) from Employees

Sum

The SUM() aggregate function returns the sum of the values selected for all rows.

SELECT SUM(Salary) FROM Employees