stddev_samp

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Returns the sample standard deviation of non-NULL values.

Syntax

stddev_samp(<expr>)

Arguments

ArgumentTypeDescription
<expr>int, uint, or floatInput value.

Returns

Returns float. Ignores NULL values. Returns NULL when fewer than two non-NULL values are available.

Examples

Estimate how much request latency varies across channels from a sample of requests:

VALUES
    ('web', 100.0), ('web', 120.0), ('web', 140.0),
    ('mobile', 60.0), ('mobile', 120.0), ('mobile', 180.0)
SELECT $0 AS channel, $1 AS latency_ms
GROUP BY channel
AGGREGATE avg(latency_ms) AS avg_ms, stddev_samp(latency_ms) AS stddev_ms
ORDER BY channel
+---------+--------+-----------+
| channel | avg_ms | stddev_ms |
+---------+--------+-----------+
| mobile  | 120.0  | 60.0      |
| web     | 120.0  | 20.0      |
+---------+--------+-----------+

Both channels average 120 ms, but mobile durations vary more. The sample standard deviation is measured in ms.