Oracle sql query Order By gives different results

Question

Output of

Query 1:

select id from users
order by case when DEVIATION_LEVEL=2863 then 1 else 2 end

is

800019  
800030  
800040  
800003  
800007  
800015  
800025  
800026....etc   

Output of Query 2:

select id from
    (select id from users
     order by case when DEVIATION_LEVEL=2863 then 1 else 2 end)
where rownum<=16;

is

800019
800030
800028
800020
800021
800018
800012
800161...etc

Why the order changes in 2nd query? Please suggest correct solution to just limit the size of the first query result.

Answer 1

The reason is that ORDER BY cannot guarantee the ordering on duplicate values .

In your query, put the DEVIATION_LEVEL in the column list of the select and then you will understand that the order is just not same when they are duplicate values.

For example,

Query 1

SQL> SELECT empno, deptno FROM emp ORDER BY CASE WHEN deptno=10 THEN 1 ELSE 2 END;

     EMPNO     DEPTNO
---------- ----------
      7782         10
      7839         10
      7934         10
      7566         20
      7654         30
      7698         30
      7900         30
      7788         20
      7369         20
      7844         30
      7876         20
      7521         30
      7499         30
      7902         20

14 rows selected.

Query 2

SQL> SELECT empno, deptno
  2  FROM
  3    (SELECT empno, deptno FROM emp ORDER BY CASE WHEN deptno=10 THEN 1 ELSE 2 END
  4    )
  5  WHERE rownum<=5;

     EMPNO     DEPTNO
---------- ----------
      7782         10
      7934         10
      7839         10
      7369         20
      7654         30

SQL>

So, the ordering in the second case when ROWNUM is applied, it is picked randomly among the similar values.

Look at the first three ordered rows:

Output 1

     EMPNO     DEPTNO
---------- ----------
      7782         10
      7839         10
      7934         10

Output 2

     EMPNO     DEPTNO
---------- ----------
      7782         10
      7934         10
      7839         10

ORDER BY deptno will not guarantee same order every time. In above query, if you want a particular order, then make an ORDER BY on another column too, ie empno.

ORDER BY empno, deptno

If you compare both the outputs, there is no guarantee that ordering will be same always since the deptno is same as 10 in all three rows. When you have similar values, and if you order them, it is just like picking them in random.

Answer 2

When you perform a SELECT query without an ORDER BY clause the order of the result is undetermined. If you want or need to have a consistent ordering behavior then use the ORDER BY clause at the top level SELECT.

There is however the exception in oracle when you are limiting the rows with the ROWNUM field. In that case the ROWNUM filter would reduce the result set before applying the order by clause, thus removing rows that should have come first.

select id from users
order by case when DEVIATION_LEVEL=2863 then 1 else 2, id;

and

select id from
    (select id from users
     order by case when DEVIATION_LEVEL=2863 then 1 else 2 end, id)
where rownum<=16;

Answer 3

ORDER BY in a subquery does not guarantee the results; SELECT * FROM table ORDER BY 1; is not same as SELECT * FROM (SELECT * FROM table ORDER BY 1);