Map(key, value) 

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

Parameters

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

Examples

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;

Result:

┌─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;

Result:

┌─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;
┌─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.

Example

Query:

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;

Result:

┌─a.keys─────────────────┐
│ ['key1','key2','key3'] │
└────────────────────────┘

┌─a.values─┐
│ [1,2,3]  │
└──────────┘

See Also

Original article

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