Use a trigger to stop an insert or update

by

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.

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