# FLATTEN function
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The FLATTEN function is used to "flatten" a nested array structure into a single flat array.

## Syntax
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```
flatten(arrayOfArrays)
```

## Arguments
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 *arrayOfArrays*  
An array of arrays.

## Return type
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The FLATTEN function returns an array.

## Example
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In this example, the input is a nested array with two inner arrays, and the output is a single flat array containing all the elements from the inner arrays. The FLATTEN function takes the nested array `[[1, 2], [3, 4]]` and combines all the elements into a single array `[1, 2, 3, 4]`.

```
SELECT flatten(array(array(1, 2), array(3, 4)));
 [1,2,3,4]
```