You can defragment the query cache to better utilize its memory with
the FLUSH QUERY CACHE statement. The statement does not remove any
queries from the cache.

The RESET QUERY CACHE statement removes all query results from the
query cache. The FLUSH TABLES statement also does this.

URL: http://dev.mysql.com/doc/refman/5.1/en/query-cache-status-and-maintenance.html

