content_image/826317.jpg
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: 

• DROP  the MERGE  table and re-create it. 

•Use ALTER TABLE tbl_name  UNION=(...)  to change the list of underlying tables. 

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: 

•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. 

<img src='content_image/826317.jpg'>

Each index must satisfy these checks: 

•The index type of the underlying table and the MERGE  table must be the same. 

## 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. 

•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: 

