Can't create a recursive relationship in MySQL - mysql

Question

I need some help, I am trying to create a recursive relation using the code below, but keep getting an error
CREATE TABLE `Employee` (
`SSN` int,
`address` varchar(50),
`salary` varchar(50),
`sex` varchar(50),
`birthDate` varchar(50),
`dependantId` int,
`supervisorId` int,
PRIMARY KEY (`SSN`),
FOREIGN KEY (`dependantId`) REFERENCES `Dependant`(`dependantId`),
FOREIGN KEY (`supervisorId`) REFERENCES `Employee`(`supervisorId`)
);
This is the error I'm getting:
#1005 - Can't create table company recursive.employee (errno: 150 "Foreign key constraint is incorrectly formed")

Answer 1

The table needs an ID column to be "referenceable" by foreign key constraints. That column musst be UNIQUE and ideally NOT NULL as well. A typical solution would look like:
CREATE TABLE Employee (
`SSN` int,
`address` varchar(50),
`salary` varchar(50),
`sex` varchar(50),
`birthDate` varchar(50),
id int not null unique, -- non nullable unique (acts as a key)
dependantId int, -- nullable reference
supervisorId int, -- nullable reference
PRIMARY KEY (`SSN`),
FOREIGN KEY (dependantId) REFERENCES Employee (id),
FOREIGN KEY (supervisorId) REFERENCES Employee (id)
);
In this case dependantId and supervisorId can be null and become references that point to the same table Employee.
As a side note, typically hierarchies only include references to the supervisor and not to the dependant. In your case the latter is somewhat redundant and won't work well if a supervisor has more than one dependant.

Answer 2

the reference should go to the SSN as it is the primary key
So you would get
CREATE TABLE `Employee` (
`SSN` int,
`address` varchar(50),
`salary` varchar(50),
`sex` varchar(50),
`birthDate` varchar(50),
`dependantId` int,
`supervisorId` int,
PRIMARY KEY (`SSN`),
FOREIGN KEY (`dependantId`) REFERENCES `Employee`(`SSN`),
FOREIGN KEY (`supervisorId`) REFERENCES `Employee`(`SSN`)
);
INSERT INTO Employee VALUES (1,'adreess1','10', 'male', '2000-01-01',NULL,NULL)
INSERT INTO Employee VALUES (2,'adreess1','10', 'male', '2000-01-01',NULL,1)
INSERT INTO Employee VALUES (3,'adreess1','10', 'male', '2000-01-01',2,1)
SELECT * FROM Employee
SSN | address | salary | sex | birthDate | dependantId | supervisorId
--: | :------- | :----- | :--- | :--------- | ----------: | -----------:
1 | adreess1 | 10 | male | 2000-01-01 | null | null
2 | adreess1 | 10 | male | 2000-01-01 | null | 1
3 | adreess1 | 10 | male | 2000-01-01 | 2 | 1
db<>fiddle here