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docs-v2/content/flux/v0/join-data/left-outer.md

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---
title: Perform a left outer join
description: >
Use [`join.left()`](/flux/v0/stdlib/join/left/) to perform an outer left join of two streams of data.
Left joins output a row for each row in the **left** data stream with data matching
from the **right** data stream. If there is no matching data in the **right**
data stream, non-group-key columns with values from the **right** data stream are _null_.
menu:
flux_v0:
name: Left outer join
parent: Join data
weight: 102
related:
- /flux/v0/join-data/troubleshoot-joins/
- /flux/v0/stdlib/join/
- /flux/v0/stdlib/join/left/
list_code_example: |
```js
import "join"
left = from(bucket: "example-bucket-1") |> //...
right = from(bucket: "example-bucket-2") |> //...
join.left(
left: left,
right: right,
on: (l, r) => l.column == r.column,
as: (l, r) => ({l with name: r.name, location: r.location}),
)
```
---
Use [`join.left()`](/flux/v0/stdlib/join/left/) to perform an left outer join of two streams of data.
Left joins output a row for each row in the **left** data stream with data matching
from the **right** data stream. If there is no matching data in the **right**
data stream, non-group-key columns with values from the **right** data stream are _null_.
{{< svg svg="static/svgs/join-diagram.svg" class="left" >}}
{{< expand-wrapper >}}
{{% expand "View table illustration of a left outer join" %}}
{{< flex >}}
{{% flex-content "third" %}}
#### left
| | | |
| :-- | :----------------------------------- | :----------------------------------- |
| r1 | <span style="color:#9b2aff">●</span> | <span style="color:#9b2aff">●</span> |
| r2 | <span style="color:#9b2aff">●</span> | <span style="color:#9b2aff">●</span> |
{{% /flex-content %}}
{{% flex-content "third" %}}
#### right
| | | |
| :-- | :----------------------------------- | :----------------------------------- |
| r1 | <span style="color:#d30971">▲</span> | <span style="color:#d30971">▲</span> |
| r3 | <span style="color:#d30971">▲</span> | <span style="color:#d30971">▲</span> |
| r4 | <span style="color:#d30971">▲</span> | <span style="color:#d30971">▲</span> |
{{% /flex-content %}}
{{% flex-content "third" %}}
#### Left outer join result
| | | | | |
| :-- | :----------------------------------- | :----------------------------------- | :----------------------------------- | :----------------------------------- |
| r1 | <span style="color:#9b2aff">●</span> | <span style="color:#9b2aff">●</span> | <span style="color:#d30971">▲</span> | <span style="color:#d30971">▲</span> |
| r2 | <span style="color:#9b2aff">●</span> | <span style="color:#9b2aff">●</span> | | |
{{% /flex-content %}}
{{< /flex >}}
{{% /expand %}}
{{< /expand-wrapper >}}
## Use join.left to join your data
1. Import the `join` package.
2. Define the **left** and **right** data streams to join:
- Each stream must have one or more columns with common values.
Column labels do not need to match, but column values do.
- Each stream should have identical [group keys](/flux/v0/get-started/data-model/#group-key).
_For more information, see [join data requirements](/flux/v0/join-data/#data-requirements)._
3. Use `join.left()` to join the two streams together.
Provide the following parameters:
- `left`: Stream of data representing the left side of the join.
- `right`: Stream of data representing the right side of the join.
- `on`: [Join predicate](/flux/v0/join-data/#join-predicate-function-on).
For example: `(l, r) => l.column == r.column`.
- `as`: [Join output function](/flux/v0/join-data/#join-output-function-as)
that returns a record with values from each input stream.
For example: `(l, r) => ({l with column1: r.column1, column2: r.column2})`.
The following example uses a filtered selection from the
[**machineProduction** sample data set](/flux/v0/stdlib/influxdata/influxdb/sample/data/#set)
as the **left** data stream and an ad-hoc table created with [`array.from()`](/flux/v0/stdlib/array/from/)
as the **right** data stream.
{{% note %}}
#### Example data grouping
The example below ungroups the **left** stream to match the grouping of the **right** stream.
After the two streams are joined together, the joined data is grouped by `stationID`.
{{% /note %}}
```js
import "array"
import "influxdata/influxdb/sample"
import "join"
left =
sample.data(set: "machineProduction")
|> filter(fn: (r) => r.stationID == "g1" or r.stationID == "g2" or r.stationID == "g3")
|> filter(fn: (r) => r._field == "oil_temp")
|> limit(n: 5)
right =
array.from(
rows: [
{station: "g1", opType: "auto", last_maintained: 2021-07-15T00:00:00Z},
{station: "g2", opType: "manned", last_maintained: 2021-07-02T00:00:00Z},
],
)
join.left(
left: left |> group(),
right: right,
on: (l, r) => l.stationID == r.station,
as: (l, r) => ({l with opType: r.opType, maintained: r.last_maintained}),
)
|> group(columns: ["stationID"])
```
{{< expand-wrapper >}}
{{% expand "View example input and output data" %}}
{{% note %}}
_`_start` and `_stop` columns have been omitted from example input and output._
{{% /note %}}
### Input
#### left {#left-input}
| _time | _measurement | stationID | _field | _value |
| :---------------------- | :----------- | :-------- | :------- | -----: |
| 2021-08-01T00:00:00Z | machinery | g1 | oil_temp | 39.1 |
| 2021-08-01T00:00:11.51Z | machinery | g1 | oil_temp | 40.3 |
| 2021-08-01T00:00:19.53Z | machinery | g1 | oil_temp | 40.6 |
| 2021-08-01T00:00:25.1Z | machinery | g1 | oil_temp | 40.72 |
| 2021-08-01T00:00:36.88Z | machinery | g1 | oil_temp | 40.8 |
| _time | _measurement | stationID | _field | _value |
| :---------------------- | :----------- | :-------- | :------- | -----: |
| 2021-08-01T00:00:00Z | machinery | g2 | oil_temp | 40.6 |
| 2021-08-01T00:00:27.93Z | machinery | g2 | oil_temp | 40.6 |
| 2021-08-01T00:00:54.96Z | machinery | g2 | oil_temp | 40.6 |
| 2021-08-01T00:01:17.27Z | machinery | g2 | oil_temp | 40.6 |
| 2021-08-01T00:01:41.84Z | machinery | g2 | oil_temp | 40.6 |
| _time | _measurement | stationID | _field | _value |
| :---------------------- | :----------- | :-------- | :------- | -----: |
| 2021-08-01T00:00:00Z | machinery | g3 | oil_temp | 41.4 |
| 2021-08-01T00:00:14.46Z | machinery | g3 | oil_temp | 41.36 |
| 2021-08-01T00:00:25.29Z | machinery | g3 | oil_temp | 41.4 |
| 2021-08-01T00:00:38.77Z | machinery | g3 | oil_temp | 41.4 |
| 2021-08-01T00:00:51.2Z | machinery | g3 | oil_temp | 41.4 |
#### right {#right-input}
| station | opType | last_maintained |
| :------ | :----- | -------------------: |
| g1 | auto | 2021-07-15T00:00:00Z |
| g2 | manned | 2021-07-02T00:00:00Z |
### Output {#example-output}
| _time | _measurement | stationID | _field | _value | opType | maintained |
| :---------------------- | :----------- | :-------- | :------- | -----: | :----- | :------------------- |
| 2021-08-01T00:00:00Z | machinery | g1 | oil_temp | 39.1 | auto | 2021-07-15T00:00:00Z |
| 2021-08-01T00:00:11.51Z | machinery | g1 | oil_temp | 40.3 | auto | 2021-07-15T00:00:00Z |
| 2021-08-01T00:00:19.53Z | machinery | g1 | oil_temp | 40.6 | auto | 2021-07-15T00:00:00Z |
| 2021-08-01T00:00:25.1Z | machinery | g1 | oil_temp | 40.72 | auto | 2021-07-15T00:00:00Z |
| 2021-08-01T00:00:36.88Z | machinery | g1 | oil_temp | 40.8 | auto | 2021-07-15T00:00:00Z |
| _time | _measurement | stationID | _field | _value | opType | maintained |
| :---------------------- | :----------- | :-------- | :------- | -----: | :----- | :------------------- |
| 2021-08-01T00:00:00Z | machinery | g2 | oil_temp | 40.6 | manned | 2021-07-02T00:00:00Z |
| 2021-08-01T00:00:27.93Z | machinery | g2 | oil_temp | 40.6 | manned | 2021-07-02T00:00:00Z |
| 2021-08-01T00:00:54.96Z | machinery | g2 | oil_temp | 40.6 | manned | 2021-07-02T00:00:00Z |
| 2021-08-01T00:01:17.27Z | machinery | g2 | oil_temp | 40.6 | manned | 2021-07-02T00:00:00Z |
| 2021-08-01T00:01:41.84Z | machinery | g2 | oil_temp | 40.6 | manned | 2021-07-02T00:00:00Z |
| _time | _measurement | stationID | _field | _value | opType | maintained |
| :---------------------- | :----------- | :-------- | :------- | -----: | :----- | :--------- |
| 2021-08-01T00:00:00Z | machinery | g3 | oil_temp | 41.4 | | |
| 2021-08-01T00:00:14.46Z | machinery | g3 | oil_temp | 41.3 | | |
| 2021-08-01T00:00:25.29Z | machinery | g3 | oil_temp | 41.4 | | |
| 2021-08-01T00:00:38.77Z | machinery | g3 | oil_temp | 41.4 | | |
| 2021-08-01T00:00:51.2Z | machinery | g3 | oil_temp | 41.4 | | |
#### Things to note about the join output
- Because the [right stream](#right-input) does not have a row with the `g3` station tag,
rows from the [left stream](#left-input) with the `g3` stationID tag include
_null_ values in columns that are populated from the right stream (`r`) in the
`as` parameter.
{{% /expand %}}
{{< /expand-wrapper >}}
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