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STDEV_SAMP function [Aggregate]

Function 

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

Syntax 

STDDEV_SAMP ( numeric-expression )

Parameters 

numeric-expression    The expression whose sample-based standard deviation is calculated over a set of rows. The expression is commonly a column name.

Usage 

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 ]^1/2

This standard deviation does not include rows where numeric-expression is NULL. It returns NULL for a group containing no rows.

Standards and compatibility 

• SQL/92    Vendor extension.

• SQL/99    SQL/foundation feature outside of core SQL. The STDDEV syntax is a vendor extension.

• Sybase    Not supported by Adaptive Server Enterprise.

See also 

Aggregate functions

Example 

The following statement lists the average and variance in the number of items per order in different time periods:

SELECT year( ship_date ) AS Year,
    quarter( ship_date ) AS Quarter,
    avg( quantity ) AS Average,
    STDDEV_SAMP( quantity ) AS Variance
FROM sales_order_items
GROUP BY Year, Quarter
ORDER BY Year, Quarter