130 lines
3.6 KiB
Markdown
130 lines
3.6 KiB
Markdown
---
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title: join.left() function
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description: >
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`join.left()` performs a left outer join on two table streams.
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menu:
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flux_v0_ref:
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name: join.left
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parent: join
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identifier: join/left
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weight: 101
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flux/v0/tags: [transformations]
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introduced: 0.172.0
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---
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<!------------------------------------------------------------------------------
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IMPORTANT: This page was generated from comments in the Flux source code. Any
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edits made directly to this page will be overwritten the next time the
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documentation is generated.
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To make updates to this documentation, update the function comments above the
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function definition in the Flux source code:
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https://github.com/influxdata/flux/blob/master/stdlib/join/join.flux#L460-L467
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Contributing to Flux: https://github.com/influxdata/flux#contributing
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Fluxdoc syntax: https://github.com/influxdata/flux/blob/master/docs/fluxdoc.md
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------------------------------------------------------------------------------->
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`join.left()` performs a left outer join on two table streams.
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The function calls `join.tables()` with the `method` parameter set to `"left"`.
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##### Function type signature
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```js
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(<-left: stream[A], as: (l: A, r: B) => C, on: (l: A, r: B) => bool, right: stream[B]) => stream[C] where A: Record, B: Record, C: Record
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```
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{{% caption %}}
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For more information, see [Function type signatures](/flux/v0/function-type-signatures/).
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{{% /caption %}}
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## Parameters
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### left
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Left input stream. Default is piped-forward data (<-).
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### right
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({{< req >}})
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Right input stream.
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### on
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({{< req >}})
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Function that takes a left and right record (`l`, and `r` respectively), and returns a boolean.
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The body of the function must be a single boolean expression, consisting of one
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or more equality comparisons between a property of `l` and a property of `r`,
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each chained together by the `and` operator.
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### as
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({{< req >}})
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Function that takes a left and a right record (`l` and `r` respectively), and returns a record.
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The returned record is included in the final output.
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## Examples
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### Perform a left outer join
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In a left outer join, `l` is guaranteed to not be a default record, but `r` may be a
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default record. Because `r` is more likely to contain null values, the output record
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is built almost entirely from properties of `l`, with the exception of `v_right`, which
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we expect to sometimes be null.
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For more information about the behavior of outer joins, see the [Outer joins](/flux/v0/stdlib/join/#outer-joins)
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section in the `join` package documentation.
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```js
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import "array"
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import "join"
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left =
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array.from(
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rows: [
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{_time: 2022-01-01T00:00:00Z, _value: 1, label: "a"},
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{_time: 2022-01-01T00:00:00Z, _value: 2, label: "b"},
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{_time: 2022-01-01T00:00:00Z, _value: 3, label: "d"},
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],
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)
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right =
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array.from(
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rows: [
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{_time: 2022-01-01T00:00:00Z, _value: 0.4, id: "a"},
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{_time: 2022-01-01T00:00:00Z, _value: 0.5, id: "c"},
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{_time: 2022-01-01T00:00:00Z, _value: 0.6, id: "d"},
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],
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)
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join.left(
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left: left,
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right: right,
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on: (l, r) => l.label == r.id and l._time == r._time,
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as: (l, r) => ({_time: l._time, label: l.label, v_left: l._value, v_right: r._value}),
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)
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```
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{{< expand-wrapper >}}
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{{% expand "View example output" %}}
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#### Output data
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| _time | label | v_left | v_right |
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| -------------------- | ------ | ------- | -------- |
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| 2022-01-01T00:00:00Z | a | 1 | 0.4 |
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| 2022-01-01T00:00:00Z | b | 2 | |
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| 2022-01-01T00:00:00Z | d | 3 | 0.6 |
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{{% /expand %}}
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{{< /expand-wrapper >}}
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