# STDDEV, STDDEV_SAMP, STDDEV_POP Functions

An aggregate function that standard deviation of a set of numbers.

**Syntax:**

{ STDDEV | STDDEV_SAMP | STDDEV_POP } ([DISTINCT | ALL]expression)

This function works with any numeric data type.

**Return type:** `DOUBLE` in Impala 2.0 and higher; `STRING` in earlier releases

This function is typically used in mathematical formulas related to probability distributions.

The `STDDEV_POP()` and `STDDEV_SAMP()` functions compute the population standard deviation and sample standard deviation,
respectively, of the input values. (`STDDEV()` is an alias for `STDDEV_SAMP()`.) Both functions evaluate all input rows matched by the query.
The difference is that `STDDEV_SAMP()` is scaled by `1/(N-1)` while `STDDEV_POP()` is scaled by `1/N`.

If no input rows match the query, the result of any of these functions is `NULL`. If a single input row matches the query, the result of any of these
functions is `"0.0"`.

**Examples:**

This example demonstrates how `STDDEV()` and `STDDEV_SAMP()` return the same result, while `STDDEV_POP()` uses a slightly different calculation to reflect that the input data is considered part of a larger "population".

[localhost:21000] > select stddev(score) from test_scores; +---------------+ | stddev(score) | +---------------+ | 28.5 | +---------------+ [localhost:21000] > select stddev_samp(score) from test_scores; +--------------------+ | stddev_samp(score) | +--------------------+ | 28.5 | +--------------------+ [localhost:21000] > select stddev_pop(score) from test_scores; +-------------------+ | stddev_pop(score) | +-------------------+ | 28.4858 | +-------------------+

This example demonstrates that, because the return value of these aggregate functions is a `STRING`, you must currently convert the result with `CAST`.

[localhost:21000] > create table score_stats as select cast(stddev(score) as decimal(7,4)) `standard_deviation`, cast(variance(score) as decimal(7,4)) `variance` from test_scores; +-------------------+ | summary | +-------------------+ | Inserted 1 row(s) | +-------------------+ [localhost:21000] > desc score_stats; +--------------------+--------------+---------+ | name | type | comment | +--------------------+--------------+---------+ | standard_deviation | decimal(7,4) | | | variance | decimal(7,4) | | +--------------------+--------------+---------+

**Restrictions:**

This function cannot be used in an analytic context. That is, the `OVER()` clause is not allowed at all
with this function.

The `STDDEV()`, `STDDEV_POP()`, and `STDDEV_SAMP()` functions compute the standard deviation
(square root of the variance) based on the results of `VARIANCE()`, `VARIANCE_POP()`, and `VARIANCE_SAMP()`
respectively. See VARIANCE, VARIANCE_SAMP, VARIANCE_POP, VAR_SAMP, VAR_POP Functions for details about the variance property.

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