How does the IN predicate work in SQL?

Question

After prepairing an answer for this question I found I couldn\'t verify my answer.

In my first programming job I was told that a query within the IN ()

Answer 1

This depends on the RDBMS in question.

See detailed analysis here:

• MySQL, part 1
• MySQL, part 2
• SQL Server
• Oracle
• PostgreSQL

In short:

1. MySQL will optimize the query to this:

   SELECT  COUNT(*)
   FROM    Table1 t1
   WHERE   NOT EXISTS
           (
           SELECT  1
           FROM    Table2 t2
           WHERE   t2.id_user = 1
                   AND t2.Table1ID = t1.Table2ID
           )
   

   and run the inner subquery in a loop, using the index lookup each time.

   • SQL Server will use MERGE ANTI JOIN.

   The inner subquery will not be "executed" in a common sense of word, instead, the results from both query and subquery will be fetched concurrently.

   See the link above for detailed explanation.

   • Oracle will use HASH ANTI JOIN.

   The inner subquery will be executed once, and a hash table will be built from the resultset.

   The values from the outer query will be looked up in the hash table.

   • PostgreSQL will use NOT (HASHED SUBPLAN).

   Much like Oracle.

Note that rewriting the query as this:

SELECT  (
        SELECT  COUNT(*)
        FROM    Table1
        ) - 
        (
        SELECT  COUNT(*)
        FROM    Table2 t2
        WHERE   (t2.id_user, t2.Table1ID) IN
                (
                SELECT  1, Table1ID
                FROM    Table1
                )
        )

will greatly improve the performance in all four systems.