VAR_SAMP function [Aggregate]

Computes the statistical variance of a sample consisting of a numeric-expression, as a DOUBLE.

VAR_SAMP ( [ALL] expression )

The following statement lists the average and variance in the number of items per order in different time periods:

SELECT year( ship_date ) AS Year, quarter( ship_date )
  AS Quarter, AVG( quantity ) AS Average,  
  VAR_POP( quantity ) AS Variance 
FROM sales_order_items GROUP BY Year, Quarter 
  ORDER BY Year, Quarter

Computes the sample variance of value expression evaluated for each row of the group or partition (if DISTINCT was specified, then each row that remains after duplicates have been eliminated), defined as the sum of squares of the difference of value expression, from the mean of value expression, divided by one less than the number of rows (remaining) in the group or partition.

Variances are computed according to the following formula, which assumes a normal distribution: