Unable to use composite key as foreign key - oracle

Question

CREATE TABLE departments
( department_id number(10) NOT NULL,
department_name varchar2(50) NOT NULL,
department_code varchar2(50) NOT NULL,
CONSTRAINT departments_pk PRIMARY KEY (department_id, department_code)
);
CREATE TABLE employees
( employee_number number(10) NOT NULL,
employee_name varchar2(50) NOT NULL,
department_id number(10),
salary number(6),
CONSTRAINT employees_pk PRIMARY KEY (employee_number),
CONSTRAINT fk_departments
FOREIGN KEY (department_id, department_code)
REFERENCES departments(department_id,department_code));

Answer 1

Your employees table doesn't have a department_code field, so the FOREIGN KEY (department_id, department_code) part is trying to use something that doesn't exist. Which is what the error you get from running the second statement tells you:
ERROR at line 8:
ORA-00904: "DEPARTMENT_CODE": invalid identifier
You would either have to include the department code in that table, which would denormalise the data; or change the primary key on departments to just department_id, which would be much more normal anyway, i.e.:
CREATE TABLE departments
( department_id number(10) NOT NULL,
department_name varchar2(50) NOT NULL,
department_code varchar2(50) NOT NULL,
CONSTRAINT departments_pk PRIMARY KEY (department_id)
);
CREATE TABLE employees
( employee_number number(10) NOT NULL,
employee_name varchar2(50) NOT NULL,
department_id number(10),
salary number(6),
CONSTRAINT employees_pk PRIMARY KEY (employee_number),
CONSTRAINT fk_departments
FOREIGN KEY (department_id)
REFERENCES departments(department_id));
It doesn't usually make sense to have a composite primary key like that, where there is a single column that looks like it should be unique anyway. You wouldn't expect to have the same department_id with two department_code values.
But if you do have a legitimate reason to have a composite primary key then all columns in that key will have to be duplicated on the child tables and their foreign key constraints:
CREATE TABLE employees
( employee_number number(10) NOT NULL,
employee_name varchar2(50) NOT NULL,
department_id number(10),
department_code varchar2(50) NOT NULL,
salary number(6),
CONSTRAINT employees_pk PRIMARY KEY (employee_number),
CONSTRAINT fk_departments
FOREIGN KEY (department_id, department_code)
REFERENCES departments(department_id,department_code));
Table EMPLOYEES created.
Which means that whenever you insert a record into that table you will have to supply both the ID and code for an existing department, of course.