# How can I take standard deviation in MYsql?

I have two columns containing two different timestamps for particular user. How can I calculate Standard deviation for this.

Is there any direct function available to perform this like sum() or avg()
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Software DeveloperCommented:
You have the STDDEV() function... Not sure if that's what you need: http://dev.mysql.com/doc/refman/5.1/en/group-by-functions.html#function_stddev
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Commented:
Look into the STD(), or  STDDEV_POP() aggregate functions in mysql.

http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_std
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Senior Principal Technical Support EngineerCommented:
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Author Commented:
Thanks guys for reply.....All of these methods are for finding standard deviation population where one value is randomly picked from table and then used as average..

I s there any function which can give regualr standard deviation whch is generally found by calculating exact mean
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Senior Principal Technical Support EngineerCommented:
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Commented:
mysql supports STD() which is population standard deviation, and STDDEV_SAMP() which returns the sample standard deviation. Not sure but maybe the sample deviation is what you're looking for.

Alternatively you can do it manually using some variables. You can nest the aggregate functions in MySQL But I think if you get the average first and store it in a variable, you can do the rest. E.g. something like:

SELECT @ln_average := AVG(total) FROM yourtable;
SELECT (1/COUNT(total)) * SUM( POW(total - @ln_average,2) )  FROM yourtable;
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Commented:
"You can nest the aggregate functions in MySQL"

Sorry, you CAN'T nest the aggregate functions in MySQL.
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Author Commented:
I don't  think there is any direct fuction . so I am going by mathematic expression and calculate result out of it
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