
Map Functions and Operators
===========================

Subscript Operator: \[\]
------------------------

The `[]` operator is used to retrieve the value corresponding to a given key from a map:

    SELECT name_to_age_map['Bob'] AS bob_age;

Map Functions
-------------

**cardinality(x)** -\> bigint

Returns the cardinality (size) of the map `x`.



**element\_at(map(K,V), key)** -\> V

Returns value for given `key`, or `NULL` if the key is not contained in the map.


**map()** -\> map\<unknown, unknown\>

Returns an empty map. :

    SELECT map(); -- {}


**map(array(K), array(V))** -\> map(K,V)

Returns a map created using the given key/value arrays. :

    SELECT map(ARRAY[1,3], ARRAY[2,4]); -- {1 -> 2, 3 -> 4}

See also `map_agg` and `multimap_agg` for creating a map as an aggregation.

**map\_from\_entries(array(row(K,V)))** -\> map(K,V)

Returns a map created from the given array of entries. :

    SELECT map_from_entries(ARRAY[(1, 'x'), (2, 'y')]); -- {1 -> 'x', 2 -> 'y'}

**multimap\_from\_entries(array(row(K,V)))** -\> map(K,array(V))

Returns a multimap created from the given array of entries. Each key can be associated with multiple values. :

    SELECT multimap_from_entries(ARRAY[(1, 'x'), (2, 'y'), (1, 'z')]); -- {1 -> ['x', 'z'], 2 -> ['y']}

**map\_entries(map(K,V))** -\> array(row(K,V))

Returns an array of all entries in the given map. :

    SELECT map_entries(MAP(ARRAY[1, 2], ARRAY['x', 'y'])); -- [ROW(1, 'x'), ROW(2, 'y')]

**map\_concat(map1(K,V), map2(K,V), \..., mapN(K,V))** -\> map(K,V)

Returns the union of all the given maps. If a key is found in multiple given maps, that key\'s value in the resulting map comes from the last one of those maps.

**map\_filter(map(K,V), function(K,V,boolean))** -\> map(K,V)

Constructs a map from those entries of `map` for which `function` returns true:

    SELECT map_filter(MAP(ARRAY[], ARRAY[]), (k, v) -> true); -- {}
    SELECT map_filter(MAP(ARRAY[10, 20, 30], ARRAY['a', NULL, 'c']), (k, v) -> v IS NOT NULL); -- {10 -> a, 30 -> c}
    SELECT map_filter(MAP(ARRAY['k1', 'k2', 'k3'], ARRAY[20, 3, 15]), (k, v) -> v > 10); -- {k1 -> 20, k3 -> 15}


**map\_keys(x(K,V))** -\> array(K)

Returns all the keys in the map `x`.


**map\_values(x(K,V))** -\> array(V)

Returns all the values in the map `x`.


**map\_zip\_with(map(K,V1), map(K,V2), function(K,V1,V2,V3))** -\> map(K,V3)

Merges the two given maps into a single map by applying `function` to the pair of values with the same key. For keys only presented in one map, NULL will be passed as the value for the missing key. :

    SELECT map_zip_with(MAP(ARRAY[1, 2, 3], ARRAY['a', 'b', 'c']), -- {1 -> ad, 2 -> be, 3 -> cf}
                        MAP(ARRAY[1, 2, 3], ARRAY['d', 'e', 'f']),
                        (k, v1, v2) -> concat(v1, v2));
    SELECT map_zip_with(MAP(ARRAY['k1', 'k2'], ARRAY[1, 2]), -- {k1 -> ROW(1, null), k2 -> ROW(2, 4), k3 -> ROW(null, 9)}
                        MAP(ARRAY['k2', 'k3'], ARRAY[4, 9]),
                        (k, v1, v2) -> (v1, v2));
    SELECT map_zip_with(MAP(ARRAY['a', 'b', 'c'], ARRAY[1, 8, 27]), -- {a -> a1, b -> b4, c -> c9}
                        MAP(ARRAY['a', 'b', 'c'], ARRAY[1, 2, 3]),
                        (k, v1, v2) -> k || CAST(v1/v2 AS VARCHAR));


**transform\_keys(map(K1,V), function(K1,V,K2))** -\> map(K2,V)

Returns a map that applies `function` to each entry of `map` and transforms the keys:

    SELECT transform_keys(MAP(ARRAY[], ARRAY[]), (k, v) -> k + 1); -- {}
    SELECT transform_keys(MAP(ARRAY [1, 2, 3], ARRAY ['a', 'b', 'c']), (k, v) -> k + 1); -- {2 -> a, 3 -> b, 4 -> c}
    SELECT transform_keys(MAP(ARRAY ['a', 'b', 'c'], ARRAY [1, 2, 3]), (k, v) -> v * v); -- {1 -> 1, 4 -> 2, 9 -> 3}
    SELECT transform_keys(MAP(ARRAY ['a', 'b'], ARRAY [1, 2]), (k, v) -> k || CAST(v as VARCHAR)); -- {a1 -> 1, b2 -> 2}
    SELECT transform_keys(MAP(ARRAY [1, 2], ARRAY [1.0, 1.4]), -- {one -> 1.0, two -> 1.4}
                          (k, v) -> MAP(ARRAY[1, 2], ARRAY['one', 'two'])[k]);


**transform\_values(map(K,V1), function(K,V1,V2))** -\> map(K,V2)

Returns a map that applies `function` to each entry of `map` and transforms the values:

    SELECT transform_values(MAP(ARRAY[], ARRAY[]), (k, v) -> v + 1); -- {}
    SELECT transform_values(MAP(ARRAY [1, 2, 3], ARRAY [10, 20, 30]), (k, v) -> v + k); -- {1 -> 11, 2 -> 22, 3 -> 33}
    SELECT transform_values(MAP(ARRAY [1, 2, 3], ARRAY ['a', 'b', 'c']), (k, v) -> k * k); -- {1 -> 1, 2 -> 4, 3 -> 9}
    SELECT transform_values(MAP(ARRAY ['a', 'b'], ARRAY [1, 2]), (k, v) -> k || CAST(v as VARCHAR)); -- {a -> a1, b -> b2}
    SELECT transform_values(MAP(ARRAY [1, 2], ARRAY [1.0, 1.4]), -- {1 -> one_1.0, 2 -> two_1.4}
                            (k, v) -> MAP(ARRAY[1, 2], ARRAY['one', 'two'])[k] || '_' || CAST(v AS VARCHAR));

