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

Try the SIGNAL syntax - https://dev.mysql.com/doc/refman/5.5/en/signal.html

create trigger agency_check
before insert on foo
for each row
begin
  if (new.agency < 1 or new.agency >5) then
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'your error message';
  end if 
end

EDIT

Updated based on popular comment below by Bill Karwin.