Altering Tables and Indexes
Indexes can be added or removed from a table after creation. For example, to add an index to the customer table, you can issue the following statement:
ALTER TABLE customer ADD INDEX cities (city);
To remove an index from the customer table, use the following statement:
ALTER TABLE customer DROP INDEX names;
The ALTER TABLE statement can also be used to add, remove, and alter all other aspects of the table, such as attributes and the primary index. We don't discuss statements for altering the database in this book; many examples can be found in Section 7.8 of the manual.html file that is distributed with MySQL.