MERGE
A MERGE table is a virtual table that combines multiple MyISAM tables that have similar structure into one table. The MERGE storage engine is also known as the MRG_MyISAM engine. The MERGE table does not have its own indexes; it uses indexes of the component tables instead.
Using MERGE table, you can speed up performance when joining multiple tables. MySQL only allows you to perform SELECT, DELETE, UPDATE and INSERT operations on the MERGE tables. If you use DROP TABLE statement on a MERGE table, only MERGE specification is removed. The underlying tables will not be affected.