MYSQL on delete cascade example - mysql

Question

I can't figure out how to use on delete cascade in my example.
CREATE TABLE `users` (
`login` varchar(16) NOT NULL,
`password` varchar(16) NOT NULL,
PRIMARY KEY (`login`),
UNIQUE KEY `login` (`login`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
CREATE TABLE `employees` (
`ID` int(11) NOT NULL AUTO_INCREMENT,
`Name` varchar(20) NOT NULL,
`Surname` varchar(25) NOT NULL,
`Birthday` date NOT NULL,
`Adres` varchar(50) NOT NULL,
`Telephone` varchar(25) NOT NULL,
`login` varchar(16) DEFAULT NULL,
PRIMARY KEY (`ID`),
UNIQUE KEY `login` (`login`),
CONSTRAINT `employees_ibfk_1` FOREIGN KEY (`login`) REFERENCES `users` (`login`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
One table stores informations about employee and if he has account(login) there is another table with logins and passwords.
My goals:
Not every employee needs to have user account(that means - unique record in Users table) so Employees.Login shouldn't be restricted with NOT NULL.
I want possiblity to add new user in Users and then assign it to some employee.
When I delete employee, related unique user is also deleted(if he had one).
First two are already done. I don't know how to make 3 happen now.

Answer 1

You've put the CONSTRAINT in the wrong CREATE TABLE statement. It goes on the referencing table, which is "users", because that's the table you want to automatically delete from when a row in the other table is deleted.
You want to put a nullable "ID" in the "users" table, which is unique and a foreign key to "employees".

Answer 2

Use this query :
CREATE TABLE `users` (
`login` varchar(16) NOT NULL,
`password` varchar(16) NOT NULL,
PRIMARY KEY (`login`),
UNIQUE KEY `login` (`login`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
CREATE TABLE `employees` (
`ID` int(11) NOT NULL AUTO_INCREMENT,
`Name` varchar(20) NOT NULL,
`Surname` varchar(25) NOT NULL,
`Birthday` date NOT NULL,
`Adres` varchar(50) NOT NULL,
`Telephone` varchar(25) NOT NULL,
`login` varchar(16) DEFAULT NULL,
PRIMARY KEY (`ID`),
UNIQUE KEY `login` (`login`),
CONSTRAINT `employees_ibfk_1` FOREIGN KEY (`login`) REFERENCES `users` (`login`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Answer 3

Based on suggestion from Hammerite now my tables definion looks like this:
CREATE TABLE `employees` (
`ID` int(11) NOT NULL AUTO_INCREMENT,
`Name` varchar(20) NOT NULL,
`Surname` varchar(25) NOT NULL,
`Birthday` date NOT NULL,
`Adres` varchar(50) NOT NULL,
`Telephone` varchar(25) NOT NULL,
PRIMARY KEY (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
CREATE TABLE `users` (
`login` varchar(16) NOT NULL,
`password` varchar(16) NOT NULL,
`employee_id` int(11) NOT NULL,
PRIMARY KEY (`login`),
UNIQUE KEY `login` (`login`),
UNIQUE KEY `employee_id` (`employee_id`),
CONSTRAINT `users_ibfk_1` FOREIGN KEY (`employee_id`) REFERENCES `employees` (`ID`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8
I have deleted login column from employees table
I have created another column employee_id in User table. It's unique foreign key and cannot be null since I dont want user accounts that are not used by anybody.
Now If I do this:
INSERT INTO users VALUES('login123','password',2);
then this row will be deleted after I do this:
DELETE FROM employees WHERE id=2;
I was sure that I need to make some reference in employees table to users table, thats why I didn't think about solution like this.
THANKS!