You can use memcached and an RDBMS togther, though. While RDBMSs will use memory to cache query results (with varying degrees of effectiveness), it'd be unreasonable for them to try to know about opportunities for caching e.g. generated HTML. That's really not their concern. (You may wish to look into views, though.)

The database is for searching, indexing, and ensuring that transactions are atomic. The cacheing is a different layer.