Foreign KEY Using SQL/ORACLE - database

Question

I'm trying to declare a FOREIGN KEY using SQL but i'm always getting that error :
FOREIGN KEY (Dno) REFERENCES DEPARTMENT (Dnumber))
ERROR at line 14: ORA-00942: table or view does not exist
FOREIGN KEY (Mgr_ssn) REFERENCES EMPLOYEE(Ssn))
*
ERROR at line 8: ORA-00942: table or view does not exist
This is the code i'm using :
CREATE TABLE employee (fname VARCHAR (15) NOT NULL,
minit CHAR,
lname VARCHAR (15) NOT NULL,
ssn CHAR (9) NOT NULL,
bdate DATE,
address VARCHAR (30),
sex CHAR,
salary DECIMAL (10, 2),
super_ssn CHAR (9),
dno INT NOT NULL,
PRIMARY KEY (ssn),
FOREIGN KEY (super_ssn) REFERENCES employee (ssn),
FOREIGN KEY (dno) REFERENCES department (dnumber));
CREATE TABLE department (dname VARCHAR (15) NOT NULL,
dnumber INT NOT NULL,
mgr_ssn CHAR (9) NOT NULL,
mgr_start_date DATE,
PRIMARY KEY (dnumber),
UNIQUE (dname),
FOREIGN KEY (mgr_ssn) REFERENCES employee (ssn));

Answer 1

Since your employee table create statement has references to the department table, you'll need to make sure the department table is created first. However, you have a reference to the employee table as part of the create department table script. Circular references ahoy!
Fortunately, you can create constraints separately from the create table script, which is what you'll have to do in this case, e.g.:
CREATE TABLE employee (fname VARCHAR (15) NOT NULL,
minit CHAR,
lname VARCHAR (15) NOT NULL,
ssn CHAR (9) NOT NULL,
bdate DATE,
address VARCHAR (30),
sex CHAR,
salary DECIMAL (10, 2),
super_ssn CHAR (9),
dno INT NOT NULL,
PRIMARY KEY (ssn),
FOREIGN KEY (super_ssn) REFERENCES employee (ssn));
CREATE TABLE department (dname VARCHAR (15) NOT NULL,
dnumber INT NOT NULL,
mgr_ssn CHAR (9) NOT NULL,
mgr_start_date DATE,
PRIMARY KEY (dnumber),
UNIQUE (dname),
FOREIGN KEY (mgr_ssn) REFERENCES employee (ssn));
alter table employee add constraint emp_dept_fk FOREIGN KEY (dno) REFERENCES department (dnumber);