# map函数和运算符

## 下标运算符： []

`[]`运算符用于从map中检索给定键对应的值：

``````SELECT name_to_age_map['Bob'] AS bob_age;
``````

## map函数

cardinality(x) -> bigint

element_at(map(K,V), key) -> V

map() -> map<unknown, unknown>

``````SELECT map(); -- {}
``````

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

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

map_from_entries(array(row(K,V))) -> map(K,V)

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

multimap_from_entries(array(row(K,V))) -> map(K,array(V))

``````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))

``````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)

map_filter(map(K,V), function(K,V,boolean)) -> map(K,V)

``````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)

map_values(x(K,V)) -> array(V)

map_zip_with(map(K,V1), map(K,V2), function(K,V1,V2,V3)) -> map(K,V3)

``````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)

``````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)

``````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));``````