SQL max number of records - mysql

Question

So I have the following table:
CREATE TABLE Hospital_MedicalRecord(
recNo CHAR(5),
patient CHAR(9),
doctor CHAR(9),
enteredOn DATE NOT NULL,
diagnosis VARCHAR(50) NOT NULL,
treatment VARCHAR(50),
PRIMARY KEY (recNo, patient),
FOREIGN KEY (patient) REFERENCES Hospital_Patient(NINumber),
FOREIGN KEY (doctor) REFERENCES Hospital_Doctor(NINumber)
);
I want to make it so there are never more that 65,535 medical records for a single patient. Am I supposed to make a new statement or should I implement it in the table above. I can post the patient table if needed.

Answer 1

You would typically use a before insert trigger for this, that raises an error if the number of records for a patient reached the limit and a new insert is attempted:
delimiter //
create trigger Trg_Hospital_MedicalRecord
before insert on Hospital_MedicalRecord
for each row
begin
if (
select count(*) from Hospital_MedicalRecord where patient = new.patient
) = 65535 then
set msg = concat('Patient ', new.patient, ' cannot have more than 65535 records');
signal state '45000' set message_text = msg;
end if;
end
//
delimiter ;
I would assume that you should not allow a patient to be updated on an existing record. But if this may happen, then you also need a before update trigger (with the very same code).

Answer 2

Consider the following...
DROP TABLE IF EXISTS my_table;
CREATE TABLE my_table(id SERIAL PRIMARY KEY, user_id INT NOT NULL);
INSERT INTO my_table (user_id)
SELECT 1
FROM (SELECT 1) x
LEFT
JOIN (SELECT user_id FROM my_table GROUP BY user_id HAVING COUNT(*) >=3) y
ON y.user_id = 1
WHERE y.user_id IS NULL
LIMIT 1;
This limits INSERTS to 3 per user_id.