• STDDEV_POP( expr ) [ over_clause ] 

Returns the population standard deviation of expr  (the square root of VAR_POP() ). You can also use STD()  or STDDEV() , which are equivalent but not standard SQL. 

If there are no matching rows, STDDEV_POP()  returns NULL . 

This function executes as a window function if over_clause  is present. over_clause  is as described in Section 12.21.2, “Window Function Concepts and Syntax” . 

• STDDEV_SAMP( expr ) [ over_clause ] 

Returns the sample standard deviation of expr  (the square root of VAR_SAMP() . 

If there are no matching rows, STDDEV_SAMP()  returns NULL . 

This function executes as a window function if over_clause  is present. over_clause  is as described in Section 12.21.2, “Window Function Concepts and Syntax” . 

• SUM([DISTINCT] expr ) [ over_clause ] 

Returns the sum of expr . If the return set has no rows, SUM()  returns NULL . The DISTINCT  keyword can be used to sum only the distinct values of expr . 

If there are no matching rows, SUM()  returns NULL . 

This function executes as a window function if over_clause  is present. over_clause  is as described in Section 12.21.2, “Window Function Concepts and Syntax” ; it cannot be used with DISTINCT . 

• VAR_POP( expr ) [ over_clause ] 

Returns the population standard variance of expr . It considers rows as the whole population, not as a sample, so it has the number of rows as the denominator. You can also use VARIANCE() , which is equivalent but is not standard SQL. 

If there are no matching rows, VAR_POP()  returns NULL . 

This function executes as a window function if over_clause  is present. over_clause  is as described in Section 12.21.2, “Window Function Concepts and Syntax” . 

• VAR_SAMP( expr ) [ over_clause ] 

Returns the sample variance of expr . That is, the denominator is the number of rows minus one. 

If there are no matching rows, VAR_SAMP()  returns NULL . 

This function executes as a window function if over_clause  is present. over_clause  is as described in Section 12.21.2, “Window Function Concepts and Syntax” . 

• VARIANCE( expr ) [ over_clause ] 

Returns the population standard variance of expr . VARIANCE()  is a synonym for the standard SQL function VAR_POP() , provided as a MySQL extension. 

If there are no matching rows, VARIANCE()  returns NULL . 

This function executes as a window function if over_clause  is present. over_clause  is as described in Section 12.21.2, “Window Function Concepts and Syntax” . 

