Syntax

Description of the illustration approx_count.eps

Purpose

APPROX_COUNT returns the approximate count of an expression. If you supply MAX_ERROR as the second argument, then the function returns the maximum error between the actual and approximate count.

You must use this function with a corresponding APPROX_RANK function in the HAVING clause. If a query uses APPROX_COUNT, APPROX_SUM, or APPROX_RANK, then the query must not use any other aggregation functions.

Examples

The following query returns the 10 most common jobs within every department:

SELECT department_id, job_id, 
       APPROX_COUNT(*) 
FROM   employees
GROUP BY department_id, job_id
HAVING 
  APPROX_RANK ( 
  PARTITION BY department_id 
  ORDER BY APPROX_COUNT(*) 
  DESC ) <= 10;