---
title: union() function
description: >
  `union()` merges two or more input streams into a single output stream.
menu:
  flux_v0_ref:
    name: union
    parent: universe
    identifier: universe/union
weight: 101
flux/v0/tags: [transformations]
introduced: 0.7.0
---

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`union()` merges two or more input streams into a single output stream.

The output schemas of `union()` is the union of all input schemas.
`union()` does not preserve the sort order of the rows within tables.
Use `sort()` if you need a specific sort order.

### Union vs join
`union()` does not modify data in rows, but unions separate streams of tables
into a single stream of tables and groups rows of data based on existing group keys.
`join()` creates new rows based on common values in one or more specified columns.
Output rows also contain the differing values from each of the joined streams.

##### Function type signature

```js
(tables: [stream[A]]) => stream[A] where A: Record
```

{{% caption %}}
For more information, see [Function type signatures](/flux/v0/function-type-signatures/).
{{% /caption %}}

## Parameters

### tables
({{< req >}})
List of two or more streams of tables to union together.




## Examples

- [Union two streams of tables with unique group keys](#union-two-streams-of-tables-with-unique-group-keys)
- [Union two streams of tables with empty group keys](#union-two-streams-of-tables-with-empty-group-keys)

### Union two streams of tables with unique group keys

```js
import "generate"

t1 =
    generate.from(
        count: 4,
        fn: (n) => n + 1,
        start: 2022-01-01T00:00:00Z,
        stop: 2022-01-05T00:00:00Z,
    )
        |> set(key: "tag", value: "foo")
        |> group(columns: ["tag"])

t2 =
    generate.from(
        count: 4,
        fn: (n) => n * (-1),
        start: 2022-01-01T00:00:00Z,
        stop: 2022-01-05T00:00:00Z,
    )
        |> set(key: "tag", value: "bar")
        |> group(columns: ["tag"])

union(tables: [t1, t2])

```

{{< expand-wrapper >}}
{{% expand "View example output" %}}

#### Output data

| _time                | _value  | *tag |
| -------------------- | ------- | ---- |
| 2022-01-01T00:00:00Z | 0       | bar  |
| 2022-01-02T00:00:00Z | -1      | bar  |
| 2022-01-03T00:00:00Z | -2      | bar  |
| 2022-01-04T00:00:00Z | -3      | bar  |

| _time                | _value  | *tag |
| -------------------- | ------- | ---- |
| 2022-01-01T00:00:00Z | 1       | foo  |
| 2022-01-02T00:00:00Z | 2       | foo  |
| 2022-01-03T00:00:00Z | 3       | foo  |
| 2022-01-04T00:00:00Z | 4       | foo  |

{{% /expand %}}
{{< /expand-wrapper >}}

### Union two streams of tables with empty group keys

```js
import "generate"

t1 =
    generate.from(
        count: 4,
        fn: (n) => n + 1,
        start: 2021-01-01T00:00:00Z,
        stop: 2021-01-05T00:00:00Z,
    )
        |> set(key: "tag", value: "foo")
        |> group()

t2 =
    generate.from(
        count: 4,
        fn: (n) => n * (-1),
        start: 2021-01-01T00:00:00Z,
        stop: 2021-01-05T00:00:00Z,
    )
        |> set(key: "tag", value: "bar")
        |> group()

union(tables: [t1, t2])

```

{{< expand-wrapper >}}
{{% expand "View example output" %}}

#### Output data

| _time                | _value  | tag  |
| -------------------- | ------- | ---- |
| 2021-01-01T00:00:00Z | 1       | foo  |
| 2021-01-02T00:00:00Z | 2       | foo  |
| 2021-01-03T00:00:00Z | 3       | foo  |
| 2021-01-04T00:00:00Z | 4       | foo  |
| 2021-01-01T00:00:00Z | 0       | bar  |
| 2021-01-02T00:00:00Z | -1      | bar  |
| 2021-01-03T00:00:00Z | -2      | bar  |
| 2021-01-04T00:00:00Z | -3      | bar  |

{{% /expand %}}
{{< /expand-wrapper >}}