Is there an alternative to performing a check constraint with subqueries? - mysql

Question

I have two tables
Table Room(
capacity INTEGER,
roomID varchar(5)
)
and
Event(
attendance INTEGER,
room varchar(5),
CHECK(attendance <= (SELECT R.capacity FROM Room R, WHERE R.roomID = room))
)
But I guess MySQL doesn't allow for subqueries inside of checks.
Is there an alternative way to perform this check? I only have a passing familiarity with triggers, but it seems like they only allow for explicit actions like insert, delete, etc. I just want to prevent the data from being inserted unless it fits the criteria.

Answer 1

MySQL doesn't enforce CHECK constraints.
In some cases, you can use a foreign key constraint instead of a CHECK constraint. For example, let's say you wanted to constrain room.capacity to values between 15 and 100. In other dbms, I'd write a CHECK constraint; for MySQL, I'd use another table and a foreign key constraint.
create table capacities (
capacity integer,
primary key (capacity)
);
-- Assumes that all the values between 15 and 100
-- are valid capacities. But that's unlikely.
insert into capacities values
(15), (16), (17), (18),
-- . . .
(100);
create table room(
capacity INTEGER,
roomID varchar(5),
primary key (roomID),
foreign key (capacity) references capacities (capacity)
);
In your case, I think you're stuck with writing triggers. First, the table.
create table event (
eventID integer not null,
attendance integer not null,
roomID varchar(5) not null,
primary key (eventID),
foreign key (roomID) references room (roomID)
);
One trigger handles INSERT statements. I didn't look hard; there's probably a better SQLSTATE than '22003'.
delimiter $$
create trigger check_capacity_on_insert
before insert on event
for each row begin
if(new.attendance) > (select capacity from room where room.roomid = roomid) then
signal sqlstate '22003' set message_text = 'Value out of range for room.capacity';
end if;
end;
$$
delimiter ;
And another trigger handles UPDATE statements.
delimiter $$
create trigger check_capacity_on_update
before update on event
for each row begin
if(new.attendance) > (select capacity from room where room.roomid = roomid) then
signal sqlstate '22003' set message_text = 'Value out of range for room.capacity';
end if;
end;
$$
delimiter ;