UPDATE Statement with subquerry and join

Question

I have 2 tables (say, employee and dept). employee.empno and dept.empno.
These tables are not joined.
I want to update the first name (fname) in employee WHERE employee.empno = dept.empno
My query is :
UPDATE emp SET fname='jack' WHERE exists(select * from emp,dept where emp.empno = dept.empno)
But this query updates all the records.... even where there is no corresponding record in dept
For example, emp has empno values of 10,20,30,40 and dept has empno values of 10 and 20 only,
yet all records in emp are updated.
Can someone help me ???
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Answer 1

That is because an UPDATE statemente without a WHERE clause will update all of the rows.
There are three parts to an UPDATE
UPDATE tablename
SET columnlist
WHERE
The UPDATE clause specifies the table to update.
The SET clause specifies the columns to update and the values to use to update them.
The WHERE clause specified which records in the table to update.
No WHERE clause means update all rows.
The correlated subquery in the SET clause has nothing to do with the WHERE clause that determines which rows get updated. You have to be very careful to also include a WHERE clause (that often duplicates the WHERE clause in the subquery) or you will set columns to NULL that don't get values returned from the subquery.
Alias the emp table being updated and do not include the emp table in the subquery.
Also I suggest you use 'X' (or any constant that is not actually in the table) in the EXISTS test.
UPDATE emp e SET fname='jack' WHERE exists(select 'x' from dept where e.empno = dept.empno)
WHERE exists (select 'x' from dept where e.empno = dept.empno