How to add primary and foreign key constraints correctly in Oracle?

Question

I am trying to alter tables to add the necessary primary and foreign keys, but I am really struggling to run the code without getting any errors.

The script I have written for adding PKs and FKs is the following:

ALTER TABLE PROJDEPT DROP CONSTRAINT PROJDEPT_pk;
ALTER TABLE EMPLOYEE DROP CONSTRAINT EMPLOYEE_pk DROP CONSTRAINT fk_EMPLOYEE;
ALTER TABLE PROJECT DROP CONSTRAINT PROJECT_pk DROP CONSTRAINT fk_PROJECT;
ALTER TABLE PROJECTTASK DROP CONSTRAINT PROJECTTASK_pk DROP CONSTRAINT fk_PROJECTTASK DROP CONSTRAINT fk_PROJECTTASK2;

ALTER TABLE PROJDEPT ADD CONSTRAINT PROJDEPT_pk PRIMARY KEY (Proj_Dept_ID);
ALTER TABLE EMPLOYEE ADD CONSTRAINT EMPLOYEE_pk PRIMARY KEY (Employee_ID);
ALTER TABLE PROJECT ADD CONSTRAINT PROJECT_pk PRIMARY KEY (Project_ID);
ALTER TABLE PROJECTTASK ADD CONSTRAINT PROJECTTASK_pk PRIMARY KEY (Project_ID, Employee_ID);

ALTER TABLE EMPLOYEE 
  ADD CONSTRAINT fk_EMPLOYEE 
  FOREIGN KEY (Proj_Dept_ID)
  REFERENCES PROJDEPT(Proj_Dept_ID);

ALTER TABLE PROJECT
  ADD CONSTRAINT fk_PROJECT 
  FOREIGN KEY (Proj_Dept_ID)
  REFERENCES PROJDEPT(Proj_Dept_ID);

ALTER TABLE PROJJECTTASK 
  ADD CONSTRAINT fk_PROJECTTASK FOREIGN KEY (Project_ID) REFERENCES PROJECT(Project_ID),
  ADD CONSTRAINT fk_PROJECTTASK2 FOREIGN KEY (Employee_ID) REFERENCES EMPLOYEE(Employee_ID);

The ER Diagram I am trying to put into Oracle is down below. Any help would be greatly appreciated!

Answer 1

Drop the constraints in the reverse order that you create them.

Also, you've misspelt PROJECTTASK as PROJJECTTASK .

ALTER TABLE PROJECTTASK DROP CONSTRAINT PROJECTTASK_pk
                        DROP CONSTRAINT fk_PROJECTTASK
                        DROP CONSTRAINT fk_PROJECTTASK2;

ALTER TABLE EMPLOYEE DROP CONSTRAINT EMPLOYEE_pk DROP CONSTRAINT fk_EMPLOYEE;
ALTER TABLE PROJECT DROP CONSTRAINT PROJECT_pk DROP CONSTRAINT fk_PROJECT;
ALTER TABLE PROJDEPT DROP CONSTRAINT PROJDEPT_pk;

ALTER TABLE PROJDEPT ADD CONSTRAINT PROJDEPT_pk PRIMARY KEY (Proj_Dept_ID);
ALTER TABLE PROJECT ADD CONSTRAINT PROJECT_pk PRIMARY KEY (Project_ID);
ALTER TABLE PROJECT
  ADD CONSTRAINT fk_PROJECT 
  FOREIGN KEY (Proj_Dept_ID)
  REFERENCES PROJDEPT(Proj_Dept_ID);

ALTER TABLE EMPLOYEE ADD CONSTRAINT EMPLOYEE_pk PRIMARY KEY (Employee_ID);
ALTER TABLE EMPLOYEE 
  ADD CONSTRAINT fk_EMPLOYEE 
  FOREIGN KEY (Proj_Dept_ID)
  REFERENCES PROJDEPT(Proj_Dept_ID);

ALTER TABLE PROJECTTASK ADD CONSTRAINT PROJECTTASK_pk PRIMARY KEY (Project_ID, Employee_ID);

ALTER TABLE PROJECTTASK 
  ADD CONSTRAINT fk_PROJECTTASK FOREIGN KEY (Project_ID) REFERENCES PROJECT(Project_ID);

ALTER TABLE PROJECTTASK 
  ADD CONSTRAINT fk_PROJECTTASK2 FOREIGN KEY (Employee_ID) REFERENCES EMPLOYEE(Employee_ID);

db<>fiddle here