arrayJoin function

This is a very unusual function.

Normal functions do not change a set of rows, but just change the values in each row (map). Aggregate functions compress a set of rows (fold or reduce). The arrayJoin function takes each row and generates a set of rows (unfold).

This function takes an array as an argument, and propagates the source row to multiple rows for the number of elements in the array. All the values in columns are simply copied, except the values in the column where this function is applied; it is replaced with the corresponding array value.

Example:

The arrayJoin function affects all sections of the query, including the WHERE section. Notice the result 2, even though the subquery returned 1 row.

Example:

A query can use multiple arrayJoin functions. In this case, the transformation is performed multiple times and the rows are multiplied.

Example:

Important note!

Using multiple arrayJoin with same expression may not produce expected results due to optimizations. For that cases, consider modifying repeated array expression with extra operations that do not affect join result - e.g. arrayJoin(arraySort(arr)), arrayJoin(arrayConcat(arr, []))

Example:

Note the ARRAY JOIN syntax in the SELECT query, which provides broader possibilities. ARRAY JOIN allows you to convert multiple arrays with the same number of elements at a time.

Example:

Or you can use Tuple

Example: