INSERT INTO if conditions are met - mysql

Question

A user is only meant to have up to 3 keys registered to his account at any one time. To add a new key, the user must first delete another key to "make room" for a new one.
I want this to be checked server-side, but I can't get the query to work. Here is what I tried:
IF (SELECT COUNT(serial_key_nbr)
FROM keys_table WHERE user_id = 9) <= 2
THEN INSERT INTO keys_table (user_id, serial_key_nbr)
VALUES (9, 'abc123')
How to do this?

Answer 1

You can use the below mention Script for the same:
INSERT INTO keys_table (user_id, serial_key_nbr)
SELECT 9, 'abc123' FROM DUAL
WHERE
(SELECT COUNT(serial_key_nbr)
FROM keys_table WHERE user_id = 9)<=2

Answer 2

if you want to use an if to do a conditional select then I would put it in a variable like so.
BEGIN
DECLARE var1 INT;
SELECT COUNT(serial_key_nbr) INTO var1
FROM keys_table
WHERE user_id = 9;
IF var1 <= 2
THEN
INSERT INTO keys_table (user_id, serial_key_nbr)
VALUES (9, 'abc123')
END IF;

Answer 3

A trigger might be the way to go. If a condition is met, a trigger before inserting in the table can perform an invalid operation and cause the insert operation to fail:
delimiter $$
create trigger keep_three before insert on keys_table for each row
begin
if (select count(serial_key_nbr) from keys_table where user_id = new.user_id) >= 3 then
insert into non_existent_table (non_existent_field) values (new.user_id);
end if;
end$$
delimiter ;
Ugly, but it might work.
Reference:
"MySQL Triggers: How do you abort an INSERT, UPDATE or DELETE with a trigger?"
Another solution (better I think) is to forcibly delete an entry before attepting the insert. When there are less than 3 entries, the insert procedes normally:
delimiter $$
create trigger keep_three before insert on keys_table for each row
begin
while (select count(serial_key_nbr) from keys_table where user_id = new.user_id) >= 3 do
delete from keys_table where user_id = new.user_id
-- OPTIONAL: Add an ordering criteria to define which entry is deleted first
limit 1;
end while;
end$$
delimiter ;
I think this is cleaner.
A third way (I've found it here). It will return an error message (by signaling sqlstate 45000: Unhandled user defined exception) associated with the defined condition:
delimiter $$
create trigger keep_three before insert on keys table for each row
begin
declare msg varchar(255);
declare n int default 0;
set n = (select count(serial_key_nbr) from keys_table where user_id = new.user_id);
if n >= 3 then
set msg = "INSERT failed: There must be only three entries for each user. Delete an entry first";
SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = msg;
end if;
end$$
delimiter ;
A cleaner version of my first option.