Computes the standard deviation of a sample consisting of a numeric-expression, as a DOUBLE.

Syntax 1

STDDEV_SAMP( numeric-expression )

Syntax 2

STDDEV_SAMP( numeric-expression ) OVER ( window-spec )

window-spec : see Syntax 2 instructions in the Remarks section below

Parameters

Returns

DOUBLE

Remarks

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( x_I - 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.

See also

• Aggregate functions

Standards and compatibility

Example

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;