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Map(key, value)

Map(key, value) data type stores key:value pairs.


To get the value from an a Map('key', 'value') column, use a['key'] syntax. This lookup works now with a linear complexity.


Consider the table:

CREATE TABLE table_map (a Map(String, UInt64)) ENGINE=Memory;
INSERT INTO table_map VALUES ({'key1':1, 'key2':10}), ({'key1':2,'key2':20}), ({'key1':3,'key2':30});

Select all key2 values:

SELECT a['key2'] FROM table_map;


┌─arrayElement(a, 'key2')─┐
│ 10 │
│ 20 │
│ 30 │

If there's no such key in the Map() column, the query returns zeros for numerical values, empty strings or empty arrays.

INSERT INTO table_map VALUES ({'key3':100}), ({});
SELECT a['key3'] FROM table_map;


┌─arrayElement(a, 'key3')─┐
│ 100 │
│ 0 │
┌─arrayElement(a, 'key3')─┐
│ 0 │
│ 0 │
│ 0 │

Convert Tuple to Map Type

You can cast Tuple() as Map() using CAST function:

SELECT CAST(([1, 2, 3], ['Ready', 'Steady', 'Go']), 'Map(UInt8, String)') AS map;
│ {1:'Ready',2:'Steady',3:'Go'} │

Map.keys and Map.values Subcolumns

To optimize Map column processing, in some cases you can use the keys and values subcolumns instead of reading the whole column.



CREATE TABLE t_map (`a` Map(String, UInt64)) ENGINE = Memory;

INSERT INTO t_map VALUES (map('key1', 1, 'key2', 2, 'key3', 3));

SELECT a.keys FROM t_map;

SELECT a.values FROM t_map;


│ ['key1','key2','key3'] │

│ [1,2,3] │

See Also