MaxCompute:CLUSTER_SAMPLE

Limits

Before you use window functions, take note of the following limits:

• Window functions are supported only in SELECT statements.

• A window function cannot contain nested window functions or nested aggregate functions.

• You cannot use window functions together with aggregate functions of the same level.

Syntax

boolean cluster_sample(bigint <N>) OVER ([partition_clause])
boolean cluster_sample(bigint <N>, bigint <M>) OVER ([partition_clause])

Description

• cluster_sample(bigint <N>): specifies that N random rows of data are sampled.

• cluster_sample(bigint <N>, bigint <M>): specifies that rows are sampled based on a specified ratio (M/N). The number of rows that are sampled is calculated by using the following formula: partition_row_count × M/N. partition_row_count specifies the number of rows in a partition.

Parameters

• N: required. A constant of the BIGINT type. If N is set to NULL, NULL is returned.

• M: required. A constant of the BIGINT type. If M is set to NULL, NULL is returned.

• partition_clause: optional. For more information, see frame_clause.

Return value

The return value is a value of the BOOLEAN type.

Sample data

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 (path and name) to which you upload the data file.

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

Examples

• Sample five data entries. Sample statement:

  select deptno, sal    
  		from (        
        select deptno, sal, cluster_sample(5) over () as flag
        from emp
        ) sub 
       where flag = true;

  The following result is returned:

  +------------+------------+
  | deptno     | sal        |
  +------------+------------+
  | 30         | 2850       |
  | 30         | 1500       |
  | 20         | 3000       |
  | 10         | 2450       |
  | 10         | 1300       |
  +------------+------------+

• Sample about 20% of data entries. Sample statement:

  select deptno, sal
      from (
          select deptno, sal, cluster_sample(5, 1) over () as flag
          from emp
          ) sub
      where flag = true;

  The following result is returned:

  +------------+------------+
  | deptno     | sal        |
  +------------+------------+
  | 10         | 2450       |
  | 20         | 3000       |
  | 20         | 3000       |
  +------------+------------+

• Sample about 20% of data entries in each group. Sample statement:

  select deptno, sal
      from (
          select deptno, sal, cluster_sample(5, 1) over (partition by deptno) as flag
          from emp
          ) sub
      where flag = true;

  The following result is returned:

  +------------+------------+
  | deptno     | sal        |
  +------------+------------+
  | 10         | 1300       |
  | 20         | 2975       |
  | 30         | 950        |
  +------------+------------+

Related functions

CLUSTER_SAMPLE is a window function. For more information about functions related to sum calculation and data sorting in a specified window column, see Window functions.