A multi row subquery   returns one or more rows.  Since it returns multiple values, the query must use the set comparison operators (IN,ALL,ANY).   If you use a multi row subquery with the equals comparison operators, the database will return an error if more than one row is returned.

select last_name from employees where manager_id in
(select employee_id from employees where department_id in
(select department_id from departments where location_id in
(select location_id from locations where country_id=’UK’)));

You can improve the performance of this query by having Oracle9i execute the subquery only once, then simply letting Oracle9i reference it at the appropriate points in the main query. The following code block gives a better logical idea of the work Oracle must perform to give you the result. In it, the bold text represents the common parts of the subquery that are performed only once, and the places where the subquery is referenced:

SQL> with summary as
  2  (select dname, sum(sal) as dept_total
  3   from emp, dept
  4   where emp.deptno = dept.deptno
  5   group by dname)
  6  select dname, dept_total
  7  from summary
  8  where dept_total >
  9  (select sum(dept_total) * 1/3
 10   from summary)
 11  order by dept_total desc;
DNAME                DEPT_TOTAL
——————– ———-
RESEARCH                  10875

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