IN VS EXIST IN ORACLE

1.
To determine which clause offers better performance in rule-based optimization, consider how many rows the inner query will return in comparison to the outer query. In many cases, EXISTS is better because it requires you to specify a join condition, which can invoke an INDEX scan. However, IN is often better if the results of the subquery are very small. You usually want to run the query that returns the smaller set of results first.
2.
The IN and EXISTS are logically the same. The IN clause compares values returned by the subquery and filters out rows in the outer query; the EXISTS clause compares values and filters out rows inside the subquery. In the case of NULL values, the resulting set of rows is the same.