MySQL in a Nutshell, 2nd Edition

Name

STDDEV()

Synopsis

STDDEV(expression)

This function returns the population standard deviation of the given column. 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. It returns NULL if no matching rows are found. Here is an example:

SELECT CONCAT(name_first, SPACE(1), name_last) AS rep_name,
SUM(sale_amount) AS total_sales,
COUNT(sale_amount) AS total_tickets,
AVG(sale_amount) AS avg_sale_per_ticket, 
STDDEV(sale_amount) AS standard_deviation
FROM sales 
JOIN sales_reps USING(sales_rep_id)
GROUP BY sales_rep_id;

This statement employs several aggregate functions. We use SUM() to get the total sales for each sales rep, COUNT() to retrieve the number of orders for the each, AVG() to determine the average sale, and STDDEV() to find out how much each sale made by each sales rep tends to vary from each one’s average sale. Incidentally, statistical functions return several decimal places. To return only two decimal places, you can wrap each function in FORMAT().