MySQL in a Nutshell, 2nd Edition

Name

VARIANCE()

Synopsis

VARIANCE(expression)

The variance is determined by taking the difference between each given value and the average of all values given. Each of those differences is then squared, and the results are totaled. The average of that total is then determined to get the variance. This function returns the variance of a given column, based on the rows selected as a population. It’s normally used in conjunction with a GROUP BY clause specifying a unique column, so that values are compared for each unique item separately. This function is available as of version 4.1 of MySQL. Here is an example:

SELECT CONCAT(name_first, SPACE(1), name_last) AS rep_name,
AVG(sale_amount) AS avg_sale, 
STDDEV_POP(sale_amount) AS standard_deviation,
VARIANCE(sale_amount) AS variance
FROM sales 
JOIN sales_reps USING(sales_rep_id)
GROUP BY sales_rep_id;

This SQL statement uses a few aggregate functions: AVG() to determine the average sale for each sales rep; STDDEV_POP() to determine how much each sale made by each sales rep tends to vary from each rep’s average sale; and VARIANCE() to show the variances based on the population. To comply with SQL standards, VAR_POP() could have been used instead of VARIANCE().