Select first from a subquery

Question

Hello experts,
I got a query like this:
{code}
select *
  from (select 1 as id
              ,1 as customer_id
          from dual) main_query
       join (select 1 as customer
                   ,10 as lvl
               from dual
             union
             select 1 as customer
                   ,20 as lvl
               from dual) sub_query
         on (main_query.customer_id = sub_query.customer);
{code}
What I need is for the join in sub-query to return 1 result per ID that has the highest number in column lvl.
Best regards,
Igor

Answer 1

Hi,
MAX is a good way if the problem is exactly what you said.
If you need to include several columns from the row with the highest lvl, then you'll probably want a Top-N Query .
Here's one way:
WITH got_r_num AS
SELECT deptno, ename, empno, hiredate, sal
, ROW_NUMBER () OVER ( PARTITION BY  deptno
               ORDER BY      empno DESC
       ) AS r_num
FROM    scott.emp
SELECT d.deptno, d.dname
, e.ename, e.empno, e.hiredate, e.sal
FROM scott.dept  d
JOIN got_r_num   e  ON  e.deptno  = d.deptno
WHERE e.r_num     = 1
Output:
    DEPTNO DNAME          ENAME           EMPNO HIREDATE         SAL
        10 ACCOUNTING     MILLER           7934 23-JAN-82       1300
        20 RESEARCH       FORD             7902 03-DEC-81       3000
        30 SALES          JAMES            7900 03-DEC-81        950
Of course, this works even if the problem is as simple as you said.