一个外键和两个主键

Question

I have an assignment table and an employee table. I want an assignment to be associated with an existing employee and an existing project. I have two primary keys, one from the employee table and the other from the project table. Each referencing back to the assignment table. May my foreign key constraints be separate as I show or do they have to be together and have an 'and' statement?

 create table assignment 
(asg_prj_p# number(2),
asg_emp_e#a varchar2(1),
asg_emp_e#n varchar2(5),
asg_hrs number(3),
constraint fk_project foreign key (asg_prj_p#) references project (prj_p#),
constraint fk_employee foreign key (asg_emp_e#a, asg_emp_e#n) references employee (emp_e#a, emp_e#n));

Answer 1

Your question is not fully clear to me. But the following statements will help you to solve the question.

In you assignment table you have two foreign key. One is single foriegn key: asg_prj_p# and another one is composite foreign key (asg_emp_e#a, asg_emp_e#n). The composite foreign key must refer to a composite primary key or composite Unique key columns. So in your example the (emp_e#a, emp_e#n) columns of employee table must be a composite primary key or composite unique key.

Answer 2

The foreign key constraints are independent- ie one ensures you have a row in the project table with the ID specified in your asg_prog_p field, the other does the same re the employee table and the two fields that make up a key to it. So what you have looks good, in terms of foreign key constraints.

You may then want a composite primary key using these 3 fields- that depends on whether an employee can have one or more assignments on a particular project. If one only then a composite primary key might be a good idea. Otherwise you should consider if you need another field to join the composite primary key, or you could create a new field, eg 'assignment_id' to be the primary key.