---
title: Perform an inner join
description: Use join.inner() to perform an inner join of two streams of data. Inner joins drop any rows from both input streams that do not have a matching row in the other stream.
url: https://docs.influxdata.com/flux/v0/join-data/inner/
estimated_tokens: 5344
product: Flux
version: v0
---

# Perform an inner join

Use [`join.inner()`](/flux/v0/stdlib/join/inner/) to perform an inner join of two streams of data. Inner joins drop any rows from both input streams that do not have a matching row in the other stream.

[](#view-table-illustration-of-an-inner-join)

View table illustration of an inner join

#### left

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

#### right

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

#### Inner join result

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

## Use join.inner 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.inner()` to join the two streams together. Provide the following required 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.inner(
    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

*`_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 |

#### Things to note about the join output

-   Because the [right stream](#right-input) does not have a row with the `g3` station tag, the joined output drops all rows with the `g3` stationID tag from the [left stream](#left-input). `join.inner()` drops any rows that do not have a matching row in the other data stream.

#### Related

-   [Troubleshoot join operations](/flux/v0/join-data/troubleshoot-joins/)
-   [join package](/flux/v0/stdlib/join/)
-   [join.inner() function](/flux/v0/stdlib/join/inner/)