docs-v2/content/shared/influxdb-v2/query-data/flux/median.md

Use the median() function to return a value representing the 0.5 quantile (50th percentile) or median of input data.

Select a method for calculating the median

Select one of the following methods to calculate the median:

• estimate_tdigest
• exact_mean
• exact_selector

estimate_tdigest

(Default) An aggregate method that uses a t-digest data structure to compute an accurate 0.5 quantile estimate on large data sources. Output tables consist of a single row containing the calculated median.

{{< flex >}} {{% flex-content %}} Given the following input table:

_time	_value
2020-01-01T00:01:00Z	1.0
2020-01-01T00:02:00Z	1.0
2020-01-01T00:03:00Z	2.0
2020-01-01T00:04:00Z	3.0
{{% /flex-content %}}
{{% flex-content %}}
estimate_tdigest returns:

_value
1.5
{{% /flex-content %}}
{{< /flex >}}

exact_mean

An aggregate method that takes the average of the two points closest to the 0.5 quantile value. Output tables consist of a single row containing the calculated median.

{{< flex >}} {{% flex-content %}} Given the following input table:

_time	_value
2020-01-01T00:01:00Z	1.0
2020-01-01T00:02:00Z	1.0
2020-01-01T00:03:00Z	2.0
2020-01-01T00:04:00Z	3.0
{{% /flex-content %}}
{{% flex-content %}}
exact_mean returns:

_value
1.5
{{% /flex-content %}}
{{< /flex >}}

exact_selector

A selector method that returns the data point for which at least 50% of points are less than. Output tables consist of a single row containing the calculated median.

{{< flex >}} {{% flex-content %}} Given the following input table:

_time	_value
2020-01-01T00:01:00Z	1.0
2020-01-01T00:02:00Z	1.0
2020-01-01T00:03:00Z	2.0
2020-01-01T00:04:00Z	3.0
{{% /flex-content %}}
{{% flex-content %}}
exact_selector returns:

_time	_value
2020-01-01T00:02:00Z	1.0
{{% /flex-content %}}
{{< /flex >}}

{{% note %}} The examples below use the example data variable. {{% /note %}}

Find the value that represents the median

Use the default method, "estimate_tdigest", to return all rows in a table that contain values in the 50th percentile of data in the table.

data
    |> median()

Find the average of values closest to the median

Use the exact_mean method to return a single row per input table containing the average of the two values closest to the mathematical median of data in the table.

data
    |> median(method: "exact_mean")

Find the point with the median value

Use the exact_selector method to return a single row per input table containing the value that 50% of values in the table are less than.

data
    |> median(method: "exact_selector")

Use median() with aggregateWindow()

aggregateWindow() segments data into windows of time, aggregates data in each window into a single point, and then removes the time-based segmentation. It is primarily used to downsample data.

To specify the median calculation method in aggregateWindow(), use the full function syntax:

data
  |> aggregateWindow(
    every: 5m,
    fn: (tables=<-, column) => tables |> median(method: "exact_selector"),
  )