Error: SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row:

Question

Error: SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`nclinicn`.`patients`, CONSTRAINT `fk_dept_id` FOREIGN KEY (`dept_id`) REFERENCES `departments` (`dept_id`) ON DELETE SET NULL ON UPDATE CASCADE)

Hello guys i hope you're doing well. i have project for a clinic for company .i have created a registering form for patients that are employees too in the company and one of the indexes there are a department index to choose the department of the employees. when i select the department and submit the form this error comes to me everytime and i tried to fix it but it doesn't work.

the tables are :

table name : patients.
structure :
id , patient_name, company_type, date , age , phone_number, gender , job_num , national_num , nationality, job_id (the foregin key for jobs table on delete set null on update cascade) , dept_id ((the foregin key for departments table on delete set null on update cascade), illness_id (the foregin key for illnesses table on delete set null on update cascade).

table : departments
structure :
dept_id , dept_name

i use phpmyadmin , mariadb

i make sure the data i insert is exist but still this error became. if someone can help me i'll be appreciated a lot.

Answer 1

I think: on delete set Null can cause a problem. why? because it's an ID field which should be NOT NULL AUTO_INCREMENT

Answer 2

but it works with other tables . what do you think the suitable constraint for the table?