Rules for naming objects, including tables in MySql:
http://dev.mysql.com/doc/refman/5.1/en/identifiers.html
Identifiers may begin with a digit but
unless quoted may not consist solely
of digits.
So this would be invalid:
SELECT * FROM 12345;
But the following would be valid:
SELECT * FROM `12345`;
Or if running in ANSI mode the following would work:
SET @@session.sql_mode=ANSI_QUOTES;
SELECT * FROM "12345";