[Old Pedant]

When in doubt, RTFM.

I'd much rather trust the documentation than google for what somebody else may or may not have done right.

http://dev.mysql.com/doc/refman/5.5/...ate-table.html

Samples from that page:

  | [CONSTRAINT [symbol]] FOREIGN KEY [index_name] (index_col_name,...) reference_definition
...
reference_definition:
    REFERENCES tbl_name (index_col_name,...)
      [MATCH FULL | MATCH PARTIAL | MATCH SIMPLE]
      [ON DELETE reference_option]
      [ON UPDATE reference_option]

reference_option:
    RESTRICT | CASCADE | SET NULL | NO ACTION
...

Which means, for example, you can write:

    ....
    CONSTRAINT FOREIGN KEY productid REFERENCES products(productid) ON DELETE CASCADE 
    ...

but it also means you could write

    ....
    FOREIGN KEY productid REFERENCES products(productid) ON DELETE CASCADE 
    ...

or you could write

    ....
    CONSTRAINT fk_img_prodid FOREIGN KEY productid REFERENCES products(productid) ON DELETE CASCADE 
    ...

By giving the constraint a name, you could later remove or alter it by name. Not a likely thing to do with a foreign key, which is why I seldom bother, but it just gives you more options.

So...MUCH better to get the *FULL* documentation with all possible options rather than just what one person chose to use.