STDEV Function

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STDEV Function

The STDEV function computes the standard deviation of a data set, which is the square root of the VARIANCE function. You can apply the STDEV function only to numeric columns. The next query finds the standard deviation:

SELECT STDEV(age) FROM u_pop WHERE u_pop.age > 0

As with the other aggregates, the STDEV function applies to the rows of a group when the query includes a GROUP BY clause, as this example shows:

SELECT STDEV(age) FROM u_pop GROUP BY birth WHERE STDEV(age) > 0

NULL values are ignored unless every value in the specified column is NULL. If every column value is NULL, STDEV returns a NULL for that column.

Important:

All computations for the STDEV function are performed in 32-digit precision, which should be sufficient for many sets of input data. The computation, however, loses precision or returns incorrect results when all of the input data values have 16 or more digits of precision.

You cannot use this function on columns of type DATE.

Within a SELECT Statement with GROUP BY clause, STDEV returns a zero variance for a count of 1. You can omit this special case through appropriate query construction (for example, "HAVING COUNT(*) > 1"). Otherwise, a data set that has only a few cases might block the rest of the query result.