The MERGE Storage Engine mysql>  SELECT * FROM total; +---+---------+ | a | message | +---+---------+ | 1 | Testing | | 2 | table | | 3 | t1 | | 1 | Testing | | 2 | table | | 3 | t2 | +---+---------+ To remap a  MERGE  table to a different collection of  MyISAM  tables, you can use one of the following methods: It is also possible to use  ALTER TABLE ... UNION=()  (that is, with an empty  UNION  clause) to remove all of the underlying tables. However, in this case, the table is effectively empty and inserts fail because there is no underlying table to take new rows. Such a table might be useful as a template for creating new  MERGE  tables with  CREATE TABLE ... LIKE . The underlying table definitions and indexes must conform closely to the definition of the  MERGE  table. Conformance is checked when a table that is part of a  MERGE  table is opened, not when the  MERGE  table is created. If any table fails the conformance checks, the operation that triggered the opening of the table fails. This means that changes to the definitions of tables within a  MERGE  may cause a failure when the MERGE  table is accessed. The conformance checks applied to each table are: • DROP  the  MERGE  table and re-create it. •Use  ALTER TABLE  tbl_name  UNION=(...)  to change the list of underlying tables. •The underlying table and the  MERGE  table must have the same number of columns. •The column order in the underlying table and the  MERGE  table must match. •Additionally, the specification for each corresponding column in the parent  MERGE  table and the underlying tables are compared and must satisfy these checks: •The column type in the underlying table and the  MERGE  table must be equal. •The column length in the underlying table and the  MERGE  table must be equal. •The column of the underlying table and the  MERGE  table can be  NULL . •The underlying table must have at least as many indexes as the  MERGE  table. The underlying table may have more indexes than the  MERGE  table, but cannot have fewer. Note A known issue exists where indexes on the same columns must be in identical order, in both the  MERGE  table and the underlying  MyISAM  table. See Bug #33653. Each index must satisfy these checks: •The index type of the underlying table and the  MERGE  table must be the same. •The number of index parts (that is, multiple columns within a compound index) in the index definition for the underlying table and the  MERGE  table must be the same. •For each index part: 3422