• STDDEV_POP( expr ) [ over_clause ] • STDDEV_SAMP( expr ) [ over_clause ] Aggregate Function Descriptions 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” . 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” . 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 . 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” . 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” . 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” . 2404 • VARIANCE( expr ) [ over_clause ] • VAR_SAMP( expr ) [ over_clause ] • VAR_POP( expr ) [ over_clause ] • SUM([DISTINCT]  expr ) [ over_clause ]