Exists: Each row of the outer table query, the query is substituted as a test, if the result of the query returns a non-null value is taken, the EXISTS clause returns TRUE, the result of this query trekking as outer rows, or can not, as a result.
the difference:
IN the outer and the inner table is a hash table is connected, it is to execute sub-queries.
EXISTS is the appearance of circulation, including tables and queries.
Scope:
when the query is indexed field, the primary table from Table A is greater than B, the use of higher efficiency IN subqueries, A is smaller than the opposite primary table from Table B, the use of higher efficiency EXISTS subqueries
IN has a defect determination is not NULL, NULL value field is present if so, return occurs, is preferably used because NOT EXISTS.
Here is the execution plan of SQL Server:
