# Presto 316 Documentation

## 7.17. 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));