APPROX_COUNT_DISTINCT

APPROX_COUNT_DISTINCT ( expr [, expr] )

expr

One or more expressions evaluating to a scalar value.

expr is typically the name of a column. In this case, APPROX_COUNT_DISTINCT returns an estimate of the number of distinct values in the column.

The following example populates a column with one-hundred thousand unique integers and runs SELECT COUNT(DISTINCT <column>) and APPROX_COUNT_DISTINCT on the column so you can compare the results.

DROP DATABASE IF EXISTS singlestore_docs_example;
CREATE DATABASE singlestore_docs_example;
USE singlestore_docs_example;

DELIMITER //
CREATE PROCEDURE populate_table() AS
BEGIN
  CREATE TABLE acd_t(a INT);
  FOR i IN 1..100000 LOOP
    INSERT INTO acd_t VALUES(i);
  END LOOP;
END
//
DELIMITER ;

CALL populate_table();

Get the exact count of distinct values in the column a:

SELECT COUNT(DISTINCT a) FROM acd_t;

****
+--------------------+
| COUNT(DISTINCT a)  |
+--------------------+
|             100000 |
+--------------------+

Get an estimate of the number of distinct values in the column a:

SELECT APPROX_COUNT_DISTINCT(a) FROM acd_t;

****
+--------------------------+
| APPROX_COUNT_DISTINCT(a) |
+--------------------------+
|                   100498 |
+--------------------------+

Using multiple expressions:

CREATE TABLE acd (a int, b int);

INSERT INTO acd VALUES (1, 1), (1, 2), (1, 1), (2, 1), (2, 2), (2, 2);

SELECT APPROX_COUNT_DISTINCT(a, b) from acd;
****
+-----------------------------+
| APPROX_COUNT_DISTINCT(a, b) |
+-----------------------------+
|                           4 |
+-----------------------------+
1 row in set (0.254 sec)