Map Functions and Operators

Subscript Operator: []

Description: The [] operator is used to retrieve a value corresponding to a given key from a mapping.

select age_map['li'] from (values (map(array['li','wang'],array[15,27]))) as table_age(age_map);-- 15

Map Functions

cardinality(x)

Description: Returns the cardinality of map x.

select cardinality(map(array['num1','num2'],array[11,12]));-- 2

element_at(map(K, V), key)

Description: Returns the value of key in a map. If the map does not contain the key, null is returned.

select element_at(map(array['num1','num2'],array[11,12]),'num1'); --11
select element_at(map(array['num1','num2'],array[11,12]),'num3');-- NULL

map()
Description: Returns an empty map.
```
select map();-- {}
```
map(array(K), array(V)) -> map(K, V)
Description: Returns a map based on the given key-value pair array. The map_agg() and multimap_agg() functions in the aggregate function can also be used to generate maps.
```
SELECT map(ARRAY[1,3],ARRAY[2,4]);-- {1=2, 3=4}
```
map_from_entries(array(row(K, V))) -> map(K, V)
Description: Generates a map using a given array.
```
SELECT map_from_entries(ARRAY[(1, 'x'), (2, 'y')]); --  {1=x, 2=y} 
```
multimap_from_entries(array(row(K, V))) -> map(K, array(V))
Description: Returns a composite map based on a given row array. Each key can correspond to 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))
Description: Generates a row array using the given map.
```
SELECT map_entries(MAP(ARRAY[1, 2], ARRAY['x', 'y'])); --  [{1, x}, {2, y}]
```
map_concat(map1(K, V), map2(K, V), ..., mapN(K, V))
Description: 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. In the following example, key a uses the value 10 of the last map.
```
select map_concat(map(ARRAY['a','b'],ARRAY[1,2]),map(ARRAY['a', 'c'], ARRAY[10, 20]));
       _col0       
-------------------
 {a=10, b=2, c=20} 
(1 row)
```

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

Description: Constructs a new map using only the entry that maps the given function to 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); -- {k3=15, k1=20}

map_keys(x(K, V)) -> array(K)
Description: Returns all arrays constructed by keys in a map.
```
select map_keys(map(array['num1','num2'],array[11,12])); -- [num1, num2]
```
map_values(x(K, V)) -> array(V)
Description: Returns all value-constructed arrays in a map.
```
select map_values(map(array['num1','num2'],array[11,12]));-- [11, 12]
```

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

Description: Merges two given maps into a map by applying the function to a pair of values with the same key. For keys that appear only in one map, null is 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)); 
       _col0        
--------------------
 {1=ad, 2=be, 3=cf} 
(1 row)
  
SELECT map_zip_with(MAP(ARRAY['k1','k2'],ARRAY[1,2]),Map(ARRAY['K2','k3'],ARRAY[4,9]),(k,v1,v2)->(v1,v2));  -- {k3={NULL, 9}, k1={1, NULL}, k2={2, NULL}, K2={NULL, 4}}
                                       _col0                                       
-------------------------------------------------------------------------------
{k3={NULL, 9}, k1={1, NULL}, k2={2, NULL}, K2={NULL, 4}}
(1 row)
  
 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));
       _col0        
--------------------
 {a=a1, b=b4, c=c9} 
(1 row)

transform_keys(map(K1, V), function(K1, V, K2)) -> map(K2, V)

Description: For each entry in the map, map the key value K1 to the new key value K2 and keep the corresponding value unchanged.

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, 9=3, 4=2}

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]); -- {two=1.4, one=1.0}

size(x) → bigint
Description: Returns the capacity of Map(x) x.
```
select size(map(array['num1','num2'],array[11,12])); --2
```

transform_values(map(K, V1), function(K, V2, V2)) -> map(K, V2)

Description: Maps value V1 to value V2 for each entry in the map and keeps the corresponding key unchanged.

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}

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