Select a set of random rows

Question

I don't understand how the sample clause works on the select statement. Eg. SELECT * FROM clients SAMPLE(50) should return half of all rows in the table, but it's unstable. Sometimes it returns 4, sometimes 3 and even 2 (SELECT COUNT(*) FROM clients returns 6). Why???
So, if it's really unstable, how do I select a random number of rows from a query? Is there other way?

Answer 1

As noted, the number of rows returned by SAMPLE can vary. The approach I use is to set the sample number high enough so that the minimum number of rows it returns is always higher than what you want, and then chop off the excess with a rownum filter:
sql>select empno, ename, job
  2    from emp sample (50)
  3   where rownum <= 3;
    EMPNO ENAME      JOB
     7499 ALLEN      SALESMAN
     7654 MARTIN     SALESMAN
     7839 KING       PRESIDENT
3 rows selected.
sql>/
    EMPNO ENAME      JOB
     7566 JONES      MANAGER
     7698 BLAKE      MANAGER
     7782 CLARK      MANAGER
3 rows selected.
sql>/
    EMPNO ENAME      JOB
     7499 ALLEN      SALESMAN
     7566 JONES      MANAGER
     7782 CLARK      MANAGER
3 rows selected.
sql>/
    EMPNO ENAME      JOB
     7369 SMITH      CLERK
     7521 WARD       SALESMAN
     7839 KING       PRESIDENT
3 rows selected.