Question

I have table MYsql table.

name, t1 , t2 , t3. 

there many names, and their scores.

Show would calculate the standard deviation of t1,t2,t3

Tried Select name, stddev(t1,t2,t3), it didnt work.

This works:

SELECT
  NAME, STDDEV_SAMP(t)   STD
FROM
( SELECT NAME, t1 AS t FROM test._copy
  UNION ALL 
  SELECT NAME, t2 AS t FROM  test._copy
  UNION ALL
  SELECT NAME, t3 AS t FROM  test._copy
) t
GROUP BY  NAME

Answer 1

STDEV() is an aggregation function. It takes values from many rows and yields a single result.

SELECT
  name,
  STDEV(t1)   standard_deviation_of_t1,
  STDEV(t2)   standard_deviation_of_t2,
  STDEV(t3)   standard_deviation_of_t3
FROM
  yourTable
GROUP BY
  name

-- This will give meaningful results if there are several records for the
-- same name.

Unfortunately, it doesn’t take values from several fields. Just several rows, to do that you need to re-work your data to have all the values in a single column…

SELECT
  name,
  STDEV(t)   standard_deviation_of_all_t_values,
FROM
(
  SELECT name, t1 AS t FROM yourTable
  UNION ALL
  SELECT name, t2 AS t FROM yourTable
  UNION ALL
  SELECT name, t3 as t FROM yourTable
)
  AS data
GROUP BY
  name

EDIT

Another option, instead of UNION ALL could be…

SELECT
  name,
  STDEV(CASE map.field_id WHEN 1 THEN t1 WHEN 2 THEN t2 ELSE t3 END)
FROM
  your_table
CROSS JOIN
  (SELECT 1 as field_id UNION ALL SELECT 2 UNION ALL SELECT 3) AS map
GROUP BY
  name