6.17. 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.

elementat(_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.

mapfrom_entries(_array<row<K, V>>) → map<K,V>

Returns a map created from the given array of entries.

SELECT mapfrom_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 multimapfrom_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 mapentries(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.

mapfilter(_map<K, V>, function<K, V, boolean>) → MAP<K,V>

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

SELECT mapfilter(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.

mapvalues(_x<K, V>) → array<V>

Returns all the values in the map x.

mapzip_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 mapzip_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 transformkeys(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 transformvalues(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));

原文: https://prestodb.io/docs/current/functions/map.html