Question

Can someone explain the difference between IN and EXISTS and NOT IN
and NOT EXISTS.
Because I have read that EXISTS will work better than IN and NOT EXISTS will work better than NOT IN.
The query i created is follows..

delete from tt_left t
 where t.val = 0
   and t.text in (select t1.text
                      from tt_left t1
                     where t.text = t1.text
                       and t.resulttext = t1.resulttext
                       and t.val = 0
                       and t1.val = 1);

How to convert this to EXISTS?
Is there any other better method?

Answer 1

The optimiser will treat IN and EXISTS the same (if the IN clause is not a list of constants)
That’s because it is a "semi-join"

Likewise, NOT IN and NOT EXISTS will be usually treated the same. This is an "anti-semi-join". The exception is where you have a NULL in the NOT IN subquery. This causes the NOT IN to always be false

So, performance wise there is no difference but EXISTS/NOT EXISTS will always be correct

See "NOT IN vs. NOT EXISTS vs. LEFT JOIN / IS NULL: Oracle"

Oracle‘s optimizer is able to see that NOT EXISTS, NOT IN and LEFT JOIN / IS NULL are semantically equivalent as long as the list values are declared as NOT NULL.

It uses same execution plan for all three methods, and they yield same results in same time.

In Oracle, it is safe to use any method of the three described above to select values from a table that are missing in another table.

However, if the values are not guaranteed to be NOT NULL, LEFT JOIN / IS NULL or NOT EXISTS should be used rather than NOT IN, since the latter will produce different results depending on whether or not there are NULL values in the subquery resultset.

And IN vs. JOIN vs. EXISTS: Oracle which has a similar conclusion