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arrayJoin function
This is a very unusual function.
Normal functions don’t 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.
A query can use multiple arrayJoin functions. In this case, the transformation is performed multiple times.
Note the ARRAY JOIN syntax in the SELECT query, which provides broader possibilities.
Example:
SELECT arrayJoin([1, 2, 3] AS src) AS dst, 'Hello', src
┌─dst─┬─\'Hello\'─┬─src─────┐
│ 1 │ Hello │ [1,2,3] │
│ 2 │ Hello │ [1,2,3] │
│ 3 │ Hello │ [1,2,3] │
└─────┴───────────┴─────────┘