Cannot enter data into tables with inter related foreign keys - postgresql

Question

I have created the tables with following code, but the foreign key constraints does not allow data addition. What can I do to solve this problem?
CREATE TABLE Employee(
Ssn VARCHAR(10) PRIMARY KEY NOT NULL,
BDate DATE,
FName VARCHAR(25),
MInit VARCHAR(5),
LName VARCHAR(25),
Address VARCHAR(40),
Sex VARCHAR(6),
Salary INT,
SupervisorSsn VARCHAR(10),
DNumber INT
);
CREATE TABLE
CREATE TABLE Department(
DNumber INT PRIMARY KEY NOT NULL,
DName VARCHAR(15),
MgrSsn VARCHAR(10),
MgrStartDate DATE,
NumberofEmployees INT,
CONSTRAINT Department_MgrSsn_FK FOREIGN KEY(MgrSsn) REFERENCES Employee(Ssn) ON DELETE SET DEFAULT ON UPDATE CASCADE
);
ALTER TABLE Employee
ADD CONSTRAINT Employee_SupervisorSsn_FK FOREIGN KEY(SupervisorSsn) REFERENCES Employee(Ssn) ON DELETE SET DEFAULT ON UPDATE CASCADE,
ADD CONSTRAINT Employee_DNumber_FK FOREIGN KEY(DNumber) REFERENCES Department(DNumber) ON DELETE SET DEFAULT ON UPDATE CASCADE;

Answer 1

There are several ways to do that in Postgres.
Update later
The most obvious one: insert null values firs, then update it later:
insert into department
(dnumber, dname)
values
(1, 'One'),
(2, 'Two'),
(3, 'Three');
insert into employee (ssn, fname, lname, supervisorssn, dnumber)
values
('123', 'Arthur', 'Dent', '456', 1),
('456', 'Ford', 'Prefect', null, 2),
('789', 'Zaphod', 'Beeblebrox', null, 3);
update department
set mgrssn = '456'
where dnumber in (1,2);
update department
set mgrssn = '789'
where dnumber = 3;
Online example
Deferred constraints
Make the constraints deferred, so that they will be checked at the end of the transaction, rather when running the INSERT:
ALTER TABLE department
add constraint fk_dempt2emp foreign key (mgrssn) references employee
deferrable initially deferred; --<<
Then you can insert the rows in any order you like as long as everything happens in a single transaction:
begin transaction; --<< important!
insert into department
(dnumber, dname, mgrssn)
values
(1, 'One', '456'),
(2, 'Two', '456'),
(3, 'Three', '789')
insert into employee (ssn, fname, lname, supervisorssn, dnumber)
values
('123', 'Arthur', 'Dent', '456', 1),
('456', 'Ford', 'Prefect', null, 2),
('789', 'Zaphod', 'Beeblebrox', null, 3);
commit; -- the FKs will be checked here
Do everything in a single statement
You can use a data modifying CTE to insert rows into two tables. As this is evaluated as a single statement, the constraints do not need to be deferrable.
with new_depts as (
insert into department
(dnumber, dname, mgrssn)
values
(1, 'One', '456'),
(2, 'Two', '456'),
(3, 'Three', '789')
)
insert into employee (ssn, fname, lname, supervisorssn, dnumber)
values
('123', 'Arthur', 'Dent', '456', 1),
('456', 'Ford', 'Prefect', null, 2),
('789', 'Zaphod', 'Beeblebrox', null, 3)
;
Online example