Adding Subqueries to MySQL, or What Does it Take to Have a Decision-Support Engine? ∗

MySQL is an OLTP system without the ability to handle SQL subqueries. As part of our project in query optimiza-tion, we add subquery processing to MySQL through rewrit-ing. First, we incorporate the ability to handle subqueries in the FROM clause; then rewrite queries with subqueries in the WHERE clause by moving the subquery to the FROM clause and introducing suitable conditions to link the subquery and the main query tables. The result is a complete and correct unnesting procedure for SQL queries. The idiosyncrasies of SQL semantics make some rewritings tricky. While op-timization issues are not directly addressed, the rewriting opens up the possibility of more efficient query processing in several cases