Computes the standard deviation of a sample consisting of a numeric-expression, as a DOUBLE.
STDDEV_SAMP( numeric-expression )
STDDEV_SAMP( numeric-expression ) OVER ( window-spec )
window-spec : see Syntax 2 instructions in the Remarks section below
numeric-expression The expression whose sample-based standard deviation is calculated over a set of rows. The expression is commonly a column name.
This function converts its argument to DOUBLE, and performs the computation in double-precision floating point.
The standard deviation (s) is computed according to the following formula, which assumes a normal distribution:
s = [ (1/( N - 1 )) * SUM( xI - mean( x ) )2 ]½
This standard deviation does not include rows where numeric-expression is NULL. It returns NULL for a group containing either 0 or 1 rows.
For more information about the statistical computation performed, see Mathematical formulas for the aggregate functions.
Syntax 2 represents usage as a window function in a SELECT statement. As such, elements of window-spec can be specified either in the function syntax (inline), or in conjunction with a WINDOW clause in the SELECT statement. See the window-spec definition provided in WINDOW clause.
For more information about using window functions in SELECT statements, including working examples, see Window functions.
SQL/2003 SQL foundation feature (T621) outside core SQL.
The following statement lists the average and variance in the number of items per order in different time periods:
SELECT YEAR( ShipDate ) AS Year, QUARTER( ShipDate ) AS Quarter, AVG( Quantity ) AS Average, STDDEV_SAMP( quantity ) AS Variance FROM SalesOrderItems GROUP BY Year, Quarter ORDER BY Year, Quarter;
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