Unnesting

-- unnest a list, generating 3 rows (1, 2, 3)
SELECT unnest([1, 2, 3]);
-- unnesting a struct, generating two columns (a, b)
SELECT unnest({'a': 42, 'b': 84});
-- recursive unnest of a list of structs
SELECT unnest([{'a': 42, 'b': 84}, {'a': 100, 'b': NULL}], recursive := true);
-- limit depth of recurisve unnest using max_depth
SELECT unnest([[[1, 2], [3, 4]], [[5, 6], [7, 8, 9], []], [[10, 11]]], max_depth := 2);

The unnest special function is used to unnest lists or structs by one level. The function can be used as a regular scalar function, but only in the SELECT clause. Invoking unnest with the recursive parameter will unnest lists and structs of multiple levels. The depth of unnesting can be limited using the max_depth parameter (which assumes recursive unnesting by default).

Unnesting Lists

-- unnest a list, generating 3 rows (1, 2, 3)
SELECT unnest([1, 2, 3]);
-- unnest a scalar list, generating 3 rows ((1, 10), (2, 11), (3, NULL))
SELECT unnest([1, 2, 3]), unnest([10, 11]);
-- unnest a scalar list, generating 3 rows ((1, 10), (2, 10), (3, 10))
SELECT unnest([1, 2, 3]), 10;
-- unnest a list column generated from a subquery
SELECT unnest(l) + 10 FROM (VALUES ([1, 2, 3]), ([4, 5])) tbl(l);
-- empty result
SELECT unnest([]);
-- empty result
SELECT unnest(NULL);

Using unnest on a list will emit one tuple per entry in the list. When unnest is combined with regular scalar expressions, those expressions are repeated for every entry in the list. When multiple lists are unnested in the same SELECT clause, the lists are unnested side-by-side. If one list is longer than the other, the shorter list will be padded with NULL values.

An empty list and a NULL list will both unnest to zero elements.

Unnesting Structs

-- unnesting a struct, generating two columns (a, b)
SELECT unnest({'a': 42, 'b': 84});
-- unnesting a struct, generating two columns (a, b)
SELECT unnest({'a': 42, 'b': {'x': 84}});

unnest on a struct will emit one column per entry in the struct.

Recursive Unnest

-- unnesting a list of lists recursively, generating 5 rows (1, 2, 3, 4, 5)
SELECT unnest([[1, 2, 3], [4, 5]], recursive := true);
-- unnesting a list of structs recursively, generating two rows of two columns (a, b)
SELECT unnest([{'a': 42, 'b': 84}, {'a': 100, 'b': NULL}], recursive := true);
-- unnesting a struct, generating two columns (a, b)
SELECT unnest({'a': [1, 2, 3], 'b': 88}, recursive := true);

Calling unnest with the recursive setting will fully unnest lists, followed by fully unnesting structs. This can be useful to fully flatten columns that contain lists within lists, or lists of structs. Note that lists within structs are not unnested.

Seeting the Maximum Depth of Unnesting

The max_depth parameter allows limiting the maximum depth of recursive unnesting (which is assumed by default and does not have to be specified separately). For example, unnestig to max_depth of 2 yields the following:

SELECT unnest([[[1, 2], [3, 4]], [[5, 6], [7, 8, 9], []], [[10, 11]]], max_depth := 2) AS x;

x
[1, 2]
[3, 4]
[5, 6]
[7, 8, 9]
[]
[10, 11]

Meanwhile, unnesting to max_depth of 3 results in

SELECT unnest([[[1, 2], [3, 4]], [[5, 6], [7, 8, 9], []], [[10, 11]]], max_depth := 3) AS x;

x
1
2
3
4
5
6
7
8
9
10
11

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Last modified: 2024-04-18