# Map functions and operators#

## Subscript operator: []#

The `[]` operator is used to retrieve the value corresponding to a given key from a map. This operator throws an error if the key is not contained in the map. See also `element_at` function that returns `NULL` in such case.

```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']),
MAP(ARRAY[1, 2, 3], ARRAY['d', 'e', 'f']),
(k, v1, v2) -> concat(v1, v2));
-- {1 -> ad, 2 -> be, 3 -> cf}

SELECT map_zip_with(MAP(ARRAY['k1', 'k2'], ARRAY[1, 2]),
MAP(ARRAY['k2', 'k3'], ARRAY[4, 9]),
(k, v1, v2) -> (v1, v2));
-- {k1 -> ROW(1, null), k2 -> ROW(2, 4), k3 -> ROW(null, 9)}

SELECT map_zip_with(MAP(ARRAY['a', 'b', 'c'], ARRAY[1, 8, 27]),
MAP(ARRAY['a', 'b', 'c'], ARRAY[1, 2, 3]),
(k, v1, v2) -> k || CAST(v1 / v2 AS VARCHAR));
-- {a -> a1, b -> b4, c -> c9}
```
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]),
(k, v) -> MAP(ARRAY[1, 2], ARRAY['one', 'two'])[k]);
-- {one -> 1.0, two -> 1.4}
```
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]),
(k, v) -> MAP(ARRAY[1, 2], ARRAY['one', 'two'])[k]
|| '_' || CAST(v AS VARCHAR));
-- {1 -> one_1.0, 2 -> two_1.4}
```