What is the difference between the two update statements

Question

Hi Everyone
Along with the emp table in scott kindly insert the to ins statement into emp.
Insert into EMP
(EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
Values
(8000, 'JAMES', 'CLERK', 7698, sysdate,
1000, 10, 30);
Insert into EMP
(EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
Values
(8001, 'JAMES', 'CLERK', 7698, sysdate,
1000, 10, 30);
Consider the below two update statements.
1) update emp a
set ename=(select loc
from dept b
where a.deptno=b.deptno)
where exists(select loc
from dept b
where a.deptno=b.deptno)
2) update emp a
set ename=(select loc
from dept b
where a.deptno=b.deptno)
What is the difference between the 1st and 2nd update statement? what is the use of exists clause in the first update statement ? Can u suggest in what case will the two update statements provide different results.
Thanks in advance

Answer 1

Second one updates all records in emp. If there is no record in dept then ename will be updated to null.
The first one only updates records in emp where a record in dept exists. It does not matter whether loc is non-null in dept or not. In an EXISTS query it doesn't matter if you do SELECT *, SELECT LOC, SELECT 1, SELECT NULL - Oracle just checks for existence.
So most often an update like the first one is meant to improve performance by only updating those emp records that are needed. Usually that is a good idea.
For this very specific case where all employees do belong to a department, all records in emp will be updated anyway, so for this specific case it actually makes worse performance because it does work to make an unnecessary check.
Normally the EXISTS query is good for performance, but always figure out if they are needed first ;-)