flatten function
Applies to: 
Databricks SQL Databricks Runtime
Transforms an array of arrays into a single array.
Syntax
flatten(expr)
Arguments
expr: An ARRAY of ARRAY expression.
Returns
The result matches the type of the nested arrays within expr.
Examples
SQL
> SELECT flatten(array(array(1, 2), array(3, 4)));
[1,2,3,4]