It is a common operation for tables that contain an array column to produce a new table that has a column with each individual array element of that initial column, while values of other columns are duplicated. This is the basic case of what `ARRAY JOIN` clause does.
Its name comes from the fact that it can be looked at as executing `JOIN` with an array or nested data structure. The intent is similar to the [arrayJoin](../../../sql-reference/functions/array-join.md#functions_arrayjoin) function, but the clause functionality is broader.
You can specify only one `ARRAY JOIN` clause in a `SELECT` query.
Supported types of `ARRAY JOIN` are listed below:
-`ARRAY JOIN` - In base case, empty arrays are not included in the result of `JOIN`.
-`LEFT ARRAY JOIN` - The result of `JOIN` contains rows with empty arrays. The value for an empty array is set to the default value for the array element type (usually 0, empty string or NULL).
The examples below demonstrate the usage of the `ARRAY JOIN` and `LEFT ARRAY JOIN` clauses. Let’s create a table with an [Array](../../../sql-reference/data-types/array.md) type column and insert values into it:
The next example uses the `LEFT ARRAY JOIN` clause:
``` sql
SELECT s, arr
FROM arrays_test
LEFT ARRAY JOIN arr;
```
``` text
┌─s───────────┬─arr─┐
│ Hello │ 1 │
│ Hello │ 2 │
│ World │ 3 │
│ World │ 4 │
│ World │ 5 │
│ Goodbye │ 0 │
└─────────────┴─────┘
```
## Using Aliases {#using-aliases}
An alias can be specified for an array in the `ARRAY JOIN` clause. In this case, an array item can be accessed by this alias, but the array itself is accessed by the original name. Example:
``` sql
SELECT s, arr, a
FROM arrays_test
ARRAY JOIN arr AS a;
```
``` text
┌─s─────┬─arr─────┬─a─┐
│ Hello │ [1,2] │ 1 │
│ Hello │ [1,2] │ 2 │
│ World │ [3,4,5] │ 3 │
│ World │ [3,4,5] │ 4 │
│ World │ [3,4,5] │ 5 │
└───────┴─────────┴───┘
```
Using aliases, you can perform `ARRAY JOIN` with an external array. For example:
Multiple arrays can be comma-separated in the `ARRAY JOIN` clause. In this case, `JOIN` is performed with them simultaneously (the direct sum, not the cartesian product). Note that all the arrays must have the same size by default. Example:
When specifying names of nested data structures in `ARRAY JOIN`, the meaning is the same as `ARRAY JOIN` with all the array elements that it consists of. Examples are listed below:
``` sql
SELECT s, `nest.x`, `nest.y`
FROM nested_test
ARRAY JOIN `nest.x`, `nest.y`;
```
``` text
┌─s─────┬─nest.x─┬─nest.y─┐
│ Hello │ 1 │ 10 │
│ Hello │ 2 │ 20 │
│ World │ 3 │ 30 │
│ World │ 4 │ 40 │
│ World │ 5 │ 50 │
└───────┴────────┴────────┘
```
This variation also makes sense:
``` sql
SELECT s, `nest.x`, `nest.y`
FROM nested_test
ARRAY JOIN `nest.x`;
```
``` text
┌─s─────┬─nest.x─┬─nest.y─────┐
│ Hello │ 1 │ [10,20] │
│ Hello │ 2 │ [10,20] │
│ World │ 3 │ [30,40,50] │
│ World │ 4 │ [30,40,50] │
│ World │ 5 │ [30,40,50] │
└───────┴────────┴────────────┘
```
An alias may be used for a nested data structure, in order to select either the `JOIN` result or the source array. Example:
``` sql
SELECT s, `n.x`, `n.y`, `nest.x`, `nest.y`
FROM nested_test
ARRAY JOIN nest AS n;
```
``` text
┌─s─────┬─n.x─┬─n.y─┬─nest.x──┬─nest.y─────┐
│ Hello │ 1 │ 10 │ [1,2] │ [10,20] │
│ Hello │ 2 │ 20 │ [1,2] │ [10,20] │
│ World │ 3 │ 30 │ [3,4,5] │ [30,40,50] │
│ World │ 4 │ 40 │ [3,4,5] │ [30,40,50] │
│ World │ 5 │ 50 │ [3,4,5] │ [30,40,50] │
└───────┴─────┴─────┴─────────┴────────────┘
```
Example of using the [arrayEnumerate](../../../sql-reference/functions/array-functions.md#array_functions-arrayenumerate) function:
``` sql
SELECT s, `n.x`, `n.y`, `nest.x`, `nest.y`, num
FROM nested_test
ARRAY JOIN nest AS n, arrayEnumerate(`nest.x`) AS num;
The query execution order is optimized when running `ARRAY JOIN`. Although `ARRAY JOIN` must always be specified before the [WHERE](../../../sql-reference/statements/select/where.md)/[PREWHERE](../../../sql-reference/statements/select/prewhere.md) clause in a query, technically they can be performed in any order, unless result of `ARRAY JOIN` is used for filtering. The processing order is controlled by the query optimizer.
