MaxCompute:APPROX_DISTINCT

Usage notes

• New data types: If the column uses a new data type (TINYINT, SMALLINT, INT, FLOAT, VARCHAR, TIMESTAMP, or BINARY), enable the MaxCompute V2.0 data type edition first:

  • Session level: Add set odps.sql.type.system.odps2=true; before your SQL statement and submit them together.

  • Project level: Run the following statement. The setting takes effect after 10 to 15 minutes.

    setproject odps.sql.type.system.odps2=true;

    For more information about setproject, see Project operations. For precautions about enabling the MaxCompute V2.0 data type edition at the project level, see Data type editions.

• Memory overflow: Running multiple aggregate functions in a single SQL statement may cause memory overflow if project resources are insufficient. To avoid this, optimize the SQL statement or purchase additional computing resources.

Syntax

approx_distinct(<colname>)

Parameters

Return value

Returns a BIGINT value. The result has an error of 5%.

Examples

The examples in this section use a sample table named emp. To set up the table, run the following statements:

create table if not exists emp
   (empno bigint,
    ename string,
    job string,
    mgr bigint,
    hiredate datetime,
    sal bigint,
    comm bigint,
    deptno bigint);
tunnel upload emp.txt emp; -- Replace emp.txt with the actual path to your data file.

The emp.txt file contains the following data:

7369,SMITH,CLERK,7902,1980-12-17 00:00:00,800,,20
7499,ALLEN,SALESMAN,7698,1981-02-20 00:00:00,1600,300,30
7521,WARD,SALESMAN,7698,1981-02-22 00:00:00,1250,500,30
7566,JONES,MANAGER,7839,1981-04-02 00:00:00,2975,,20
7654,MARTIN,SALESMAN,7698,1981-09-28 00:00:00,1250,1400,30
7698,BLAKE,MANAGER,7839,1981-05-01 00:00:00,2850,,30
7782,CLARK,MANAGER,7839,1981-06-09 00:00:00,2450,,10
7788,SCOTT,ANALYST,7566,1987-04-19 00:00:00,3000,,20
7839,KING,PRESIDENT,,1981-11-17 00:00:00,5000,,10
7844,TURNER,SALESMAN,7698,1981-09-08 00:00:00,1500,0,30
7876,ADAMS,CLERK,7788,1987-05-23 00:00:00,1100,,20
7900,JAMES,CLERK,7698,1981-12-03 00:00:00,950,,30
7902,FORD,ANALYST,7566,1981-12-03 00:00:00,3000,,20
7934,MILLER,CLERK,7782,1982-01-23 00:00:00,1300,,10
7948,JACCKA,CLERK,7782,1981-04-12 00:00:00,5000,,10
7956,WELAN,CLERK,7649,1982-07-20 00:00:00,2450,,10
7956,TEBAGE,CLERK,7748,1982-12-30 00:00:00,1300,,10

Example 1: Count the approximate number of distinct values in the sal column.

select approx_distinct(sal) from emp;

Result:

+-------------------+
| numdistinctvalues |
+-------------------+
| 12                |
+-------------------+

Example 2: Group employees by department and count the approximate number of distinct salary values per department.

select deptno, approx_distinct(sal) from emp group by deptno;

Result:

+------------+-------------------+
| deptno     | numdistinctvalues |
+------------+-------------------+
| 10         | 3                 |
| 20         | 4                 |
| 30         | 5                 |
+------------+-------------------+

Related functions

APPROX_DISTINCT is an aggregate function. For other aggregate functions, see Aggregate functions.