Employee

Attribute	Data Type	Primary	Foreign	Constraint
SSN Number	Charr(9)	Check		Not Null
First Name	VarChar(15)			Not Null
MidName	Char
Last Name	VarChar(15)			Not Null
Birthday	Date
Gender	Char			Gender CHECK ('M','F','m','f')
Salary	Money			DEFAULT '80000'
Supervisor SSN	Char(9)		employee(SSN)
Department Number	Int

Department

Attribute	Data Type	Primary	Foreign	Constraint
Department Name	VarChar(15)			Not Null
Department Number	Int	Check		Not Null
Manager SSN	Char(9)		Employee(SSN) ON DELETE SET NULL	Not Null
Manage Start Date	Date

I need to make a foreign key with Manager SSN that relates to Employee SSN, but every time I try to put it in SQL it doesn't allow it due to referential integrity constraints.

My Code in MySQL is as follows:

Alter Table Department_JCS Add Constraint Department_fk Foreign Key (ManagerSSN) References Employee_JCS(EmployeeSSN) On Delete Set Null Go

And the problem I get is:

Msg 1761, Level 16, State 1, Server DESKTOP-54BQ7EN\LOCALDB#743D77CD, Line 1 Cannot create the foreign key "Department_fk" with the SET NULL referential action, because one or more referencing columns are not nullable.

What must I change in order to allow the constraint?