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Unnesting

Examples​

Unnest a list, generating 3 rows (1, 2, 3):

SQL
SELECT unnest([1, 2, 3]);
 unnest
--------
      1
      2
      3

Unnesting a struct, generating two columns (a, b):

SQL
SELECT unnest({'a': 42, 'b': 84});
 a  | b
----+----
 42 | 84

Recursive unnest of a list of structs:

SQL
SELECT unnest([{'a': 42, 'b': 84}, {'a': 100, 'b': NULL}], recursive := true);
 a   | b
-----+------
  42 |   84
 100 | NULL

Limit depth of recursive unnest using max_depth:

SQL
SELECT unnest([[[1, 2], [3, 4]], [[5, 6], [7, 8, 9], []], [[10, 11]]], max_depth := 2);
 unnest
---------
 {1,2}
 {3,4}
 {5,6}
 {7,8,9}
 {}
 {10,11}

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):

SQL
SELECT unnest([1, 2, 3]);
 unnest
--------
      1
      2
      3

Unnest a list, generating 3 rows ((1, 10), (2, 10), (3, 10)):

SQL
SELECT unnest([1, 2, 3]), 10;
 unnest | ?column?
--------+----------
      1 |       10
      2 |       10
      3 |       10

Unnest two lists of different sizes, generating 3 rows ((1, 10), (2, 11), (3, NULL)):

SQL
SELECT unnest([1, 2, 3]), unnest([10, 11]);
 unnest | unnest
--------+--------
      1 |     10
      2 |     11
      3 |   NULL

Unnest a list column from a subquery:

SQL
SELECT unnest(l) + 10 FROM (VALUES ([1, 2, 3]), ([4, 5])) tbl(l);
 ?column?
----------
       11
       12
       13
       14
       15

Empty result:

SQL
SELECT unnest([]);
unnest

Empty result:

SQL
SELECT unnest(NULL);
unnest

Using unnest on a list emits one row per list entry. Regular scalar expressions in the same SELECT clause are repeated for every emitted row. 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 is padded with NULL values.

Empty and NULL lists both unnest to zero rows.

Unnesting Structs​

Unnesting a struct, generating two columns (a, b):

SQL
SELECT unnest({'a': 42, 'b': 84});
 a  | b
----+----
 42 | 84

Unnesting a struct, generating two columns (a, b):

SQL
SELECT unnest({'a': 42, 'b': {'x': 84}});
 a  | b
----+------
 42 | (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):

SQL
SELECT unnest([[1, 2, 3], [4, 5]], recursive := true);
 unnest
--------
      1
      2
      3
      4
      5

Unnesting a list of structs recursively, generating two rows of two columns (a, b):

SQL
SELECT unnest([{'a': 42, 'b': 84}, {'a': 100, 'b': NULL}], recursive := true);
 a   | b
-----+------
  42 |   84
 100 | NULL

Unnesting a struct, generating two columns (a, b):

SQL
SELECT unnest({'a': [1, 2, 3], 'b': 88}, recursive := true);
 a       | b
---------+----
 {1,2,3} | 88

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.

Setting 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, unnesting to max_depth of 2 yields the following:

SQL
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:

SQL
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

Keeping Track of List Entry Positions​

To keep track of each entry's position within the original list, unnest may be combined with generate_subscripts:

SQL
SELECT unnest(l) AS x, generate_subscripts(l, 1) AS indexFROM (VALUES ([1, 2, 3]), ([4, 5])) tbl(l);
 x | index
---+-------
 1 |     1
 2 |     2
 3 |     3
 4 |     1
 5 |     2

Keep Column Names When Recursively Unnesting​

The keep_parent_names parameter can be used to retain the parent column names when recursively unnesting a named struct. For example, unnesting the following query with keep_parent_names enabled:

SQL
SELECT unnest([{'a': 0, 'b': {'bb': {'bbb': 1}}}], recursive := true, keep_parent_names := true);
 a | b.bb.bbb
---+----------
 0 |        1

In this case, the field names are preserved, showing the path to the innermost value. This is particularly useful when working with complex nested data structures, as it maintains the structure and naming convention of the original data. The parameter can also be used in conjunction with the max_depth parameter, allowing more control and enabling more precise management of nested structures.

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