added median and percential/quantile guides, related to #770
Scott Anderson
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---
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title: Query median values
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seotitle: Query median values in Flux
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description: >
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placeholder
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weight: 201
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menu:
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v2_0:
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parent: Common queries
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v2.0/tags: [query, median]
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related:
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- /v2.0/query-data/common-queries/query-percentile/
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---
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Use the [`median()` function](/v2.0/reference/flux/stdlib/built-in/transformations/aggregates/median/)
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to return all values within the `0.5` quantile (50th percentile) or the median value of input data.
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## Median calculation methods
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Select from the following methods for calculating the median:
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##### estimate_tdigest
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**(Default)** An aggregate method that uses a [t-digest data structure](https://github.com/tdunning/t-digest)
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to compute an accurate quantile estimate on large data sources.
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Returns all values in the `0.5` quantile or 50th percentile.
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##### exact_mean
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An aggregate method that takes the average of the two points closest to the quantile value.
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Output tables consist of a single row containing the calculated median.
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##### exact_selector
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A selector method that returns the data point for which at least 50% of points are less than.
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Output tables consist of a single row containing the calculated median.
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{{% note %}}
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#### Example data variable
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To focus on using the `median()` function, the examples below use a `data` variable
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which represents a base queried dataset.
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```js
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data = from(bucket: "example-bucket")
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|> range(start: -1h)
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|> filter(fn: (r) =>
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r._measurement == "example-measurement" and
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r._field == "example-field"
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)
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```
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{{% /note %}}
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## Query all values in the 50th percentile
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Use the default method, `"estimate_tdigest"`, to return all rows in a table that
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contain values in the 50th percentile of data in the table.
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```js
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data
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|> median()
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```
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## Query the average of values closest to the median
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Use the `exact_mean` method to return a single row per input table containing the
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average of the two values closest to the mathematical median of data in the table.
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```js
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data
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|> median(method: "exact_mean")
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```
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## Query the median value
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Use the `exact_selector` method to return a single row per input table containing the
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value that 50% of values in the table are less than.
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```js
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data
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|> median(method: "exact_selector")
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```
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## Use median with aggregateWindow()
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[`aggregateWindow()`](/v2.0/reference/flux/stdlib/built-in/transformations/aggregates/aggregatewindow/)
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segments data into windows of time, aggregates data in each window into a single
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point, then removes the time-based segmentation.
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It is primarily used to [downsample data](/v2.0/process-data/common-tasks/downsample-data/).
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`aggregateWindow()` expects a single point from each time window.
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Use either the `exact_mean` or `exact_mode` median calculation method.
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To specify parameters of the aggregate function in `aggregateWindow()`, use the
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[full function syntax](/v2.0/reference/flux/stdlib/built-in/transformations/aggregates/aggregatewindow/#specify-parameters-of-the-aggregate-function):
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```js
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data
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|> aggregateWindow(
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every: 5m,
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fn: (tables=<-, column) => tables |> median(method: "exact_selector")
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)
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```
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@ -0,0 +1,102 @@
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---
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title: Query percentile values
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seotitle: Query percentile values in Flux
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description: >
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placeholder
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weight: 201
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menu:
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v2_0:
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parent: Common queries
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v2.0/tags: [query, percentile, quantile]
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related:
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- /v2.0/query-data/common-queries/query-median/
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---
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Use the [`quantile()` function](/v2.0/reference/flux/stdlib/built-in/transformations/aggregates/quantile/)
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to return all values within the `q` quantile or percentile of input data.
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## Percentile versus quantile
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Percentiles and quantiles are very similar, differing only in the number used to calculate return values.
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A percentile is calculated using numbers between `0` and `100`.
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A quantile is calculated using numbers between `0.0` and `1.0`.
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For example, the **`0.5` quantile** is the same as the **50th percentile**.
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## Quantile calculation methods
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Select from the following methods for calculating the quantile:
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##### estimate_tdigest
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(Default) An aggregate method that uses a [t-digest data structure](https://github.com/tdunning/t-digest)
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to compute an accurate quantile estimate on large data sources.
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Returns all values in the `q` quantile.
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##### exact_mean
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An aggregate method that takes the average of the two points closest to the quantile value.
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Output tables consist of a single row containing the calculated quantile.
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##### exact_selector
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A selector method that returns the data point for which at least `q` points are less than.
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Output tables consist of a single row containing the calculated quantile.
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{{% note %}}
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#### Example data variable
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To focus on using the `quantile()` function, the examples below use a `data` variable
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which represents a base queried dataset.
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```js
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data = from(bucket: "example-bucket")
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|> range(start: -1h)
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|> filter(fn: (r) =>
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r._measurement == "example-measurement" and
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r._field == "example-field"
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)
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```
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{{% /note %}}
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## Query all values in the 50th percentile
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Use the default method, `"estimate_tdigest"`, to return all rows in a table that
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contain values in the 50th percentile of data in the table.
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```js
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data
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|> quantile(q: 0.99)
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```
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## Query the average of values closest to the quantile
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Use the `exact_mean` method to return a single row per input table containing the
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average of the two values closest to the mathematical quantile of data in the table.
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```js
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data
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|> quantile(q: 0.99, method: "exact_mean")
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```
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## Query the quantile value
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Use the `exact_selector` method to return a single row per input table containing the
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value that 50% of values in the table are less than.
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```js
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data
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|> quantile(q: 0.99, method: "exact_selector")
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```
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## Use quantile with aggregateWindow()
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[`aggregateWindow()`](/v2.0/reference/flux/stdlib/built-in/transformations/aggregates/aggregatewindow/)
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segments data into windows of time, aggregates data in each window into a single
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point, then removes the time-based segmentation.
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It is primarily used to [downsample data](/v2.0/process-data/common-tasks/downsample-data/).
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`aggregateWindow()` expects a single point from each time window.
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Use either the `exact_mean` or `exact_mode` quantile calculation method.
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To specify parameters of the aggregate function in `aggregateWindow()`, use the
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[full function syntax](/v2.0/reference/flux/stdlib/built-in/transformations/aggregates/aggregatewindow/#specify-parameters-of-the-aggregate-function):
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```js
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data
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|> aggregateWindow(
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every: 5m,
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fn: (tables=<-, column) =>
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tables
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|> quantile(q: 0.99, method: "exact_selector")
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)
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```
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