141 lines
3.5 KiB
Markdown
141 lines
3.5 KiB
Markdown
---
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title: keep() function
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description: The `keep()` function returns a table containing only the specified columns.
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aliases:
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- /influxdb/v2.0/reference/flux/functions/transformations/keep
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- /influxdb/v2.0/reference/flux/functions/built-in/transformations/keep/
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- /influxdb/v2.0/reference/flux/stdlib/built-in/transformations/keep/
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- /influxdb/cloud/reference/flux/stdlib/built-in/transformations/keep/
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menu:
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flux_0_x_ref:
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name: keep
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parent: universe
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weight: 102
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flux/v0.x/tags: [transformations]
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introduced: 0.7.0
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---
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The `keep()` function returns a table containing only the specified columns, ignoring all others.
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Only columns in the group key that are also specified in the `keep()` function will be kept in the resulting group key.
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_It is the inverse of [`drop`](/flux/v0.x/stdlib/universe/drop)._
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```js
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keep(columns: ["col1", "col2"])
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// OR
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keep(fn: (column) => column =~ /inodes*/)
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```
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## Parameters
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{{% note %}}
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Make sure `fn` parameter names match each specified parameter. To learn why, see [Match parameter names](/flux/v0.x/spec/data-model/#match-parameter-names).
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{{% /note %}}
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### columns {data-type="array of strings"}
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Columns that should be included in the resulting table.
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Cannot be used with `fn`.
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### fn {data-type="function"}
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A predicate function which takes a column name as a parameter (`column`) and returns
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a boolean indicating whether or not the column should be included in the resulting table.
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Cannot be used with `columns`.
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### tables {data-type="stream of tables"}
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Input data.
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Default is piped-forward data ([`<-`](/flux/v0.x/spec/expressions/#pipe-expressions)).
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## Examples
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{{% flux/sample-example-intro plural=true %}}
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- [Keep a list of columns](#keep-a-list-of-columns)
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- [Keep columns matching a predicate](#keep-columns-matching-a-predicate)
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#### Keep a list of columns
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```js
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import "sampledata"
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sampledata.int()
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|> keep(columns: ["_time", "_value"])
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```
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{{< expand-wrapper >}}
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{{% expand "View input and output" %}}
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{{< flex >}}
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{{% flex-content %}}
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##### Input data
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{{% flux/sample "int" %}}
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{{% /flex-content %}}
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{{% flex-content %}}
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##### Output data
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| _time | _value |
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| :------------------- | -----: |
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| 2021-01-01T00:00:00Z | -2 |
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| 2021-01-01T00:00:10Z | 10 |
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| 2021-01-01T00:00:20Z | 7 |
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| 2021-01-01T00:00:30Z | 17 |
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| 2021-01-01T00:00:40Z | 15 |
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| 2021-01-01T00:00:50Z | 4 |
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| 2021-01-01T00:00:00Z | 19 |
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| 2021-01-01T00:00:10Z | 4 |
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| 2021-01-01T00:00:20Z | -3 |
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| 2021-01-01T00:00:30Z | 19 |
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| 2021-01-01T00:00:40Z | 13 |
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| 2021-01-01T00:00:50Z | 1 |
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{{% /flex-content %}}
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{{< /flex >}}
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{{% /expand %}}
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{{< /expand-wrapper >}}
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#### Keep columns matching a predicate
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```js
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import "sampledata"
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sampledata.int()
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|> keep(fn: (column) => column =~ /^_?t/)
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```
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{{< expand-wrapper >}}
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{{% expand "View input and output" %}}
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{{< flex >}}
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{{% flex-content %}}
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##### Input data
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{{% flux/sample "int" %}}
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{{% /flex-content %}}
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{{% flex-content %}}
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##### Output data
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| _time | tag |
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| :------------------- | :-- |
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| 2021-01-01T00:00:00Z | t1 |
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| 2021-01-01T00:00:10Z | t1 |
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| 2021-01-01T00:00:20Z | t1 |
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| 2021-01-01T00:00:30Z | t1 |
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| 2021-01-01T00:00:40Z | t1 |
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| 2021-01-01T00:00:50Z | t1 |
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| _time | tag |
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| :------------------- | :-- |
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| 2021-01-01T00:00:00Z | t2 |
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| 2021-01-01T00:00:10Z | t2 |
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| 2021-01-01T00:00:20Z | t2 |
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| 2021-01-01T00:00:30Z | t2 |
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| 2021-01-01T00:00:40Z | t2 |
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| 2021-01-01T00:00:50Z | t2 |
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{{% /flex-content %}}
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{{< /flex >}}
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{{% /expand %}}
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{{< /expand-wrapper >}}
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