STDDEV, STDDEV_SAMP, STDDEV_POP
Synopsis
STDDEV([ALL  DISTINCT [BY(collist)]] expression [%FOREACH(collist)] [%AFTERHAVING]) STDDEV_SAMP([ALL  DISTINCT [BY(collist)]] expression [%FOREACH(collist)] [%AFTERHAVING]) STDDEV_POP([ALL  DISTINCT [BY(collist)]] expression [%FOREACH(collist)] [%AFTERHAVING])
Arguments
Argument  Description 

ALL  Optional — Specifies that standard deviation functions return the standard deviation of all values for expression. This is the default if no keyword is specified. 
DISTINCT  Optional — A DISTINCT clause that specifies that standard deviation functions return the standard deviation of the distinct (unique) expression values. DISTINCT can specify a BY(collist) subclause, where collist can be a single field or a commaseparated list of fields. 
expression  Any valid expression. Usually the name of a column that contains the data values to be analyzed for standard deviation. 
%FOREACH(collist)  Optional — A column name or a commaseparated list of column names. See SELECT for further information on %FOREACH. 
%AFTERHAVING  Optional — Applies the condition found in the HAVING clause. 
Description
These three standard deviation aggregate functions return the statistical standard deviation of the distribution of the values of expression, after discarding NULL values. That is, the amount of standard deviation from the mean value of the data set, expressed as a positive number. The larger the return value, the more variation there is within the data set of values.
The STDDEV, STDDEV_SAMP (sample), and STDDEV_POP (population) functions are derived from the corresponding variance aggregate functions:
STDDEV  VARIANCE 
STDDEV_SAMP  VAR_SAMP 
STDDEV_POP  VAR_POP 
The standard deviation is the square root of the corresponding variance value. Refer to these variance aggregate functions for further details.
These standard deviation functions can be used in a SELECT query or subquery that references either a table or a view. They can appear in a SELECT list or HAVING clause alongside ordinary field values.
These standard deviation functions cannot be used in a WHERE clause. They cannot be used in the ON clause of a JOIN, unless the SELECT is a subquery.
These standard deviation functions return a value of data type NUMERIC with a precision of 36 and a scale of 17, unless expression is data type DOUBLE in which case it returns data type DOUBLE.
These functions are normally applied to a field or expression that has a numeric value. They evaluate nonnumeric values, including the empty string (''), as zero (0).
These standard deviation functions ignore NULL values in data fields. If no rows are returned by the query, or the data field value for all rows returned is NULL, they return NULL.
The standard deviation functions, like all aggregate functions, can take an optional DISTINCT clause. STDDEV(DISTINCT col1) returns the standard deviation of those col1 field values that are distinct (unique). STDDEV(DISTINCT BY(col2) col1) returns the standard deviation of the col1 field values in records where the col2 values are distinct (unique). Note however that the distinct col2 values may include a single NULL as a distinct value.
Changes Made During the Current Transaction
Like all aggregate functions, standard deviation functions always returns the current state of the data, including uncommitted changes, regardless of the current transaction’s isolation level. For further details, refer to SET TRANSACTION and START TRANSACTION.
Examples
The following example uses STDDEV to return the standard deviation in the ages of the employees in Sample.Employee, and the standard deviation in the distinct ages represented by one or more employees:
SELECT STDDEV(Age) AS AgeSD,STDDEV(DISTINCT Age) AS PerAgeSD FROM Sample.Employee
The following example uses STDDEV_POP to return the population standard deviation in the ages of the employees in Sample.Employee, and the standard deviation in the distinct ages represented by one or more employees:
SELECT STDDEV_POP(Age) AS AgePopSD,STDDEV_POP(DISTINCT Age) AS PerAgePopSD FROM Sample.Employee
See Also

Aggregate Functions overview

VARIANCE, VAR_SAMP, VAR_POP aggregate functions

AVG aggregate function

COUNT aggregate function