When you are dealing with a huge amount of data, STDEV can return the Standard Deviation of a column. You have to have a minimum of two values or the value returned will be null.

Here is how it looks in code:

Select STDEV(Salary) As SDSalary

From Employees

The above function takes a sampling of your data instead of the whole list, and returns the standard deviation.

STDEVP

If you want to work with all the data in a column to find the standard deviation, you can use STDEVP instead.

Select STDEVP(Salary) As DevSalary

From Employees

VAR

The VAR Function allows you to find the variance of a set of data from a column. Like STDEV, VAR only works with a sampling and not the whole column. And again, you must have at least two values in the column or a null will be returned.

Select VAR(Salary) As VARSalary

From Employees

VARP

To get a variance using all of the data in a column, use this handy little guy.

Select VARP(Salary) As VARPSalary

From Employees