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cardinality

Description​

Returns the number of elements in a MAP value. MAP is an unordered collection of key-value pairs, for example, {"a":1, "b":2}. One key-value pair constitutes one element. {"a":1, "b":2} contains two elements.

This function is supported from v3.0 onwards. It is the alias of map_size().

Syntax​

INT cardinality(any_map)

Parameters​

any_map: the MAP value from which you want to retrieve the number of elements.

Return value​

Returns a value of the INT value.

If the input is NULL, NULL is returned.

If a key or value in the MAP value is NULL, NULL is processed as a normal value.

Examples​

Query MAP data from a StarRocks native table​

From v3.1 onwards, StarRocks supports defining MAP columns when you create a table. This example uses table test_map, which contains the following data:

CREATE TABLE test_map(
    col_int INT,
    col_map MAP<VARCHAR(50),INT>
  )
DUPLICATE KEY(col_int);

INSERT INTO test_map VALUES
(1,map{"a":1,"b":2}),
(2,map{"c":3}),
(3,map{"d":4,"e":5});

SELECT * FROM test_map ORDER BY col_int;
+---------+---------------+
| col_int | col_map       |
+---------+---------------+
|       1 | {"a":1,"b":2} |
|       2 | {"c":3}       |
|       3 | {"d":4,"e":5} |
+---------+---------------+
3 rows in set (0.05 sec)

Obtain the number of elements in each row of the col_map column.

select cardinality(col_map) from test_map order by col_int;
+----------------------+
| cardinality(col_map) |
+----------------------+
|                    2 |
|                    1 |
|                    2 |
+----------------------+
3 rows in set (0.05 sec)

Query MAP data from data lake​

This example uses Hive table hive_map, which contains the following data:

SELECT * FROM hive_map ORDER BY col_int;
+---------+---------------+
| col_int | col_map       |
+---------+---------------+
|       1 | {"a":1,"b":2} |
|       2 | {"c":3}       |
|       3 | {"d":4,"e":5} |
+---------+---------------+

After a Hive catalog is created in your cluster, you can use this catalog and the cardinality() function to obtain the number of elements in each row of the col_map column.

SELECT cardinality(col_map) FROM hive_map ORDER BY col_int;
+----------------------+
| cardinality(col_map) |
+----------------------+
|                    2 |
|                    1 |
|                    2 |
+----------------------+
3 rows in set (0.05 sec)