Count vs exists

Question

I have a requirement as below :
Would relate to the emp and the dept table of the SCOTT schema.
Suppose there is a third table dept_lookup .
create table dept_lookup
deptno number,-----deptno references dept's deptno
emp_count
insert into dept_lookup values (10,100);
insert into dept_lookup values (20,200);
*note : not all department's count exist in this table
in the join of emp and the dept table , i have to derive an extra column in which i have to check whether a particular deptno exists in the dept_lookup table .If exists populate the emp_count  is there then populate the count else populate a default count of 0 .I may not have pictured my actual scenario clearly but this is the extract . I want to use a scalar subquery for this .
something like
case when (select count(1) from dept_lookup dl where d.depno=dl.deptno)>=1
      then dl.emp_count else 0 end
or
case when (select 1 from dept_lookup dl where exists (select 1 from dept d where d.deptno=dl.deptno)=1
then dl.emp_count else 0 end
This has to be implemented in very large tables , considering the fact that indexes are built on the deptno on each of these tables . What would be the performance impact if i compare exists with count(1). Which one is better ?
Thanks
Rahul

Answer 1

Hi,
You shoold not make a table with calculated data in it. This should be a view. See below example:
select
from
  emp
EMPNO ENAME      JOB        MGR HIREDATE      SAL    COMM DEPTNO
7369 SMITH      CLERK     7902 17-DEC-80     800             20
7499 ALLEN      SALESMAN  7698 20-FEB-81    1600     300     30
7521 WARD       SALESMAN  7698 22-FEB-81    1250     500     30
7566 JONES      MANAGER   7839 02-APR-81    2975             20
7654 MARTIN     SALESMAN  7698 28-SEP-81    1250    1400     30
7698 BLAKE      MANAGER   7839 01-MAY-81    2850             30
7782 CLARK      MANAGER   7839 09-JUN-81    2450             10
7788 SCOTT      ANALYST   7566 19-APR-87    3000             20
7839 KING       PRESIDENT      17-NOV-81    5000             10
7844 TURNER     SALESMAN  7698 08-SEP-81    1500       0     30
7876 ADAMS      CLERK     7788 23-MAY-87    1100             20
7900 JAMES      CLERK     7698 03-DEC-81     950             30
7902 FORD       ANALYST   7566 03-DEC-81    3000             20
7934 MILLER     CLERK     7782 23-JAN-82    1300             10
select
from
  dept
DEPTNO DNAME          LOC        
    10 ACCOUNTING     NEW YORK     
    20 RESEARCH       DALLAS       
    30 SALES          CHICAGO      
    40 OPERATIONS     BOSTON      
create view dept_lookup as
select
  d.deptno
  ,count(e.empno) emp_count
from
  dept d
  left join emp e on d.deptno = e.deptno
group by
  d.deptno
select
from
  dept_lookup
DEPTNO EMP_COUNT
    10         3
    20         5
    30         6
    40         0
If you are concerned bout preformance, look into Materialized views.
Regards,
Peter