MySQL supports foreign keys, which let you cross-reference related data
across tables, and foreign key constraints, which help keep this
spread-out data consistent. The essential syntax for a foreign key
constraint definition in a CREATE TABLE or ALTER TABLE statement looks
like this:

[CONSTRAINT [symbol]] FOREIGN KEY
    [index_name] (index_col_name, ...)
    REFERENCES tbl_name (index_col_name,...)
    [ON DELETE reference_option]
    [ON UPDATE reference_option]

reference_option:
    RESTRICT | CASCADE | SET NULL | NO ACTION

URL: http://dev.mysql.com/doc/refman/5.5/en/create-table-foreign-keys.html

