APPROX_COUNT_DISTINCT

Syntax

Purpose

APPROX_COUNT_DISTINCT returns the approximate number of rows that contain distinct values of expr.

This function provides an alternative to the COUNT (DISTINCT expr) function, which returns the exact number of rows that contain distinct values of expr. APPROX_COUNT_DISTINCT processes large amounts of data significantly faster than COUNT, with negligible deviation from the exact result.

For expr, you can specify a column of any scalar data type other than BFILE, BLOB, CLOB, LONG, LONG RAW, or NCLOB.

APPROX_COUNT_DISTINCT ignores rows that contain a null value for expr. This function returns a NUMBER.

Examples

The following statement returns the approximate number of rows with distinct values for manager_id:

SELECT APPROX_COUNT_DISTINCT(manager_id) AS "Active Managers"
  FROM employees;

Active Managers
---------------
             18

The following statement returns the approximate number of distinct customers for each product:

SELECT prod_id, APPROX_COUNT_DISTINCT(cust_id) AS "Number of Customers"
  FROM sales
  GROUP BY prod_id
  ORDER BY prod_id;

   PROD_ID Number of Customers
---------- -------------------
        13                2516
        14                2030
        15                2105
        16                2367
        17                2093
        18                2975
        19                2630
        20                3791
. . .