Syntax:
CHECKSUM TABLE tbl_name [, tbl_name] ... [ QUICK | EXTENDED ]

CHECKSUM TABLE reports a table checksum.

With QUICK, the live table checksum is reported if it is available, or
NULL otherwise. This is very fast. A live checksum is enabled by
specifying the CHECKSUM=1 table option when you create the table;
currently, this is supported only for MyISAM tables. See [HELP CREATE
TABLE].

With EXTENDED, the entire table is read row by row and the checksum is
calculated. This can be very slow for large tables.

If neither QUICK nor EXTENDED is specified, MySQL returns a live
checksum if the table storage engine supports it and scans the table
otherwise.

For a non-existent table, CHECKSUM TABLE returns NULL and generates a
warning.

The checksum value depends on the table row format. If the row format
changes, the checksum also changes. For example, the storage format for
VARCHAR changed between MySQL 4.1 and 5.0, so if a 4.1 table is
upgraded to MySQL 5.0, the checksum value may change.

URL: http://dev.mysql.com/doc/refman/5.1/en/checksum-table.html

