---
title: Perform a left outer join
description: Use 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.
url: https://docs.influxdata.com/flux/v0/join-data/left-outer/
estimated_tokens: 2743
product: Flux
version: v0
publisher: InfluxData
canonical: https://docs.influxdata.com/flux/v0/join-data/left-outer/
date: '2023-09-12T23:33:31-06:00'
lastmod: '2023-09-12T23:33:31-06:00'
---

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*.

[](#view-table-illustration-of-a-left-outer-join)

View table illustration of a left outer join

#### left

|   |   |   |
|---|---|---|
|r1 | ● | ● |
|r2 | ● | ● |

#### right

|   |   |   |
|---|---|---|
|r1 | ▲ | ▲ |
|r3 | ▲ | ▲ |
|r4 | ▲ | ▲ |

#### Left outer join result

|   |   |   |   |   |
|---|---|---|---|---|
|r1 | ● | ● | ▲ | ▲ |
|r2 | ● | ● |   |   |

## 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.

#### 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`.

```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"])
```

[](#view-example-input-and-output-data)

View example input and output data

> [!Note]
> *`_start` and `_stop` columns have been omitted from example input and output.*

### Input

#### left

|        \_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

|station|opType|  last\_maintained  |
|-------|------|--------------------|
|  g1   | auto |2021-07-15T00:00:00Z|
|  g2   |manned|2021-07-02T00:00:00Z|

### 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.

#### Related

* [Troubleshoot join operations](/flux/v0/join-data/troubleshoot-joins/)
* [join package](/flux/v0/stdlib/join/)
* [join.left() function](/flux/v0/stdlib/join/left/)
|  |  |  |
| --- | --- | --- |
|  |  |  |
| r1 | ● | ● |
| r2 | ● | ● |

|  |  |  |
| --- | --- | --- |
|  |  |  |
| r1 | ▲ | ▲ |
| r3 | ▲ | ▲ |
| r4 | ▲ | ▲ |

|  |  |  |  |  |
| --- | --- | --- | --- | --- |
|  |  |  |  |  |
| r1 | ● | ● | ▲ | ▲ |
| r2 | ● | ● |  |  |

| _time | _measurement | stationID | _field | _value |
| --- | --- | --- | --- | --- |
| _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 |
| --- | --- | --- | --- | --- |
| _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 |
| --- | --- | --- | --- | --- |
| _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 |

| station | opType | last_maintained |
| --- | --- | --- |
| station | opType | last_maintained |
| g1 | auto | 2021-07-15T00:00:00Z |
| g2 | manned | 2021-07-02T00:00:00Z |

| _time | _measurement | stationID | _field | _value | opType | maintained |
| --- | --- | --- | --- | --- | --- | --- |
| _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 |
| --- | --- | --- | --- | --- | --- | --- |
| _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 |
| --- | --- | --- | --- | --- | --- | --- |
| _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 |  |  |