Use a trigger to stop an insert or update

Question

Since MySQL ignores check constraints, how does one go about using a trigger to stop an insert or update from happening?

For example:

Table foo has an attrib

Answer 1

If your version of MySQL is older than 5.5, try setting a non-null field of the table to NULL. It is a hack, but it does prevent the update or insert from completing.

The SIGNAL command which Naveen suggests looks great, and I'm looking forward to using it after we upgrade.