drop table if exists foo;
create table foo( v varchar(32) );
drop trigger if exists foo_ins;
delimiter go
create trigger foo_ins before insert on foo
for each row
begin
 if new.v = '' then
  signal sqlstate '45000' 
  set message_text="Your Insert was blocked.";
 end if;
end;
go
delimiter ;
insert into foo values(""); -- ERROR 1644 (45000): Your Insert was blocked.
insert into foo values ("abc");
select * from foo;
+------+
| v    |
+------+
| abc  |
+------+

Last updated 8 Mar 2025