What is the difference between exists and in

Question

hi all
if i have these queries
1- select ename from emp where ename in ( select ename from emp where empno=10)
and
2- select ename from emp where exists ( select ename from emp where empno=10)
what is the difference between exists and in is that only when i use in i have to bring the field name or what.... i mean in a complex SQL queries is it will give the same answer
Thanks

Answer 1

You get two entirely different result sets that may be the same. Haah! What do I mean by that.
SQL> select table_name from user_tables;
TABLE_NAME
BAR
FOO
2 rows selected.
SQL> select table_name from user_tables where table_name in (select table_name from user_tables where table_name = 'FOO');
TABLE_NAME
FOO
1 row selected.
SQL> select table_name from user_tables where exists(select table_name from user_tables where table_name = 'FOO');
TABLE_NAME
BAR
FOO
2 rows selected.So, why is this? the WHERE EXISTS means 'if the next is true', much like where 1=1 being always true and 1=2 being always false. In this case, where exists could be TRUE or FALSE, depending on the subquery.
WHERE EXISTS can be useful for something like testing if we have data, without actually having to return columns.
So, if you want to see if an employee exists you might say
SELECT 1 FROM DUAL WHERE EXISTS( select * from emp where empid = 10);
If there is a row in emp for empid=10, then you get back 1 from dual;
This is what I call an 'optimistic' lookup because the WHERE EXISTS ends as soon as there is a hit. It does not care how many - only that at least one exists. It is optimistic because it will continue processing the table lookup until either it hits or reaches the end of the table - for a non-indexed query.