How to establish many-many relationship in a oracle database? - oracle11g

Question

I'm intended to develop a database model for my department. I found it difficult to establish the relationship between student, courses and staffs considering that any number of students can elect any number of courses and any number of staffs can handle any number of courses. How will I be able to represent this data in an oracle database?

Answer 1

What did you manage to do so far? What kind of difficulties did you meet?
Anyway: here's a suggestion, see whether it helps. An example is based on your STUDENT and COURSES tables. Idea is to include additional "cross" table which maps courses and students, i.e. contains columns that make primary keys of both tables, they are constrained by foreign key constraints and both of them make the primary key of the new, cross table.
Here's the code:
Create tables:
SQL> -- Students
SQL> create table t_student
2 (id_student number constraint pk_stu primary key,
3 student_name varchar2(20) not null
4 );
Table created.
SQL> -- Courses
SQL> create table t_course
2 (id_course number constraint pk_cou primary key,
3 course_name varchar2(20) not null
4 );
Table created.
SQL> -- Additional "cross" table
SQL> create table t_stu_x_cou
2 (id_student number constraint fk_sxc_stu
3 references t_student (id_student),
4 id_course number constraint fk_sxc_cou
5 references t_course (id_course),
6 constraint pk_sxc primary key (id_student, id_course)
7 );
Table created.
Insert sample data:
SQL> insert into t_student (id_student, student_name)
2 select 1, 'Little' from dual union
3 select 2, 'Foot' from dual;
2 rows created.
SQL> insert into t_course (id_course, course_name)
2 select 100, 'Mathematics' from dual union
3 select 200, 'Physics' from dual union
4 select 300, 'Chemistry' from dual;
3 rows created.
SQL> -- Mapping students and courses:
SQL> -- - student 1 takes 2 courses (100 and 300)
SQL> -- - student 2 takes 3 courses (100, 200 and 300)
SQL> insert into t_stu_x_cou (id_student, id_course)
2 select 1, 100 from dual union
3 select 1, 300 from dual union
4 --
5 select 2, 100 from dual union
6 select 2, 200 from dual union
7 select 2, 300 from dual;
5 rows created.
Select that shows courses taken by student 1:
SQL> select s.student_name, c.course_name
2 from t_stu_x_cou x
3 join t_student s on s.id_student = x.id_student
4 join t_course c on c.id_course = x.id_course
5 where s.id_student = 1;
STUDENT_NAME COURSE_NAME
-------------------- --------------------
Little Mathematics
Little Chemistry
SQL>
Now, try to add the STAFF table yourself, using the same principle (you'd add a new "cross" table between STAFF and COURSES).