InterSystems SQL Reference
STDDEV, STDDEV_SAMP, STDDEV_POP


Aggregate functions that return the statistical standard deviation of a data set.
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])
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:
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.
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