Re: [SQL] Database Design question

At 09:39 +0300 on 31/05/1999, Heiko Wilms wrote:

> trying to redisign a database with the ER-model, I now have some N:N
> relations.
> I solved this by using arrays in some cases but querying gets very
> complex then.
> What is a "common" way to treat N:N relations?

You create a third table, which describes the relation. The classical
example is a relation between students and courses. Each student can study
in several courses, and there are also several students in each course.

Thus you have:
1. Student table, which contains all data which is only student-related.
(such as first and last name, status, etc.)
2. Course table, containing all course data (such as syllabus)
3. Student-course table, sometimes called "enrollment" table. It contains
the key into the student table, the key into the course table,
and perhaps extra details which relate to the specific enrollment,
such as the student's current mark in the given course.

(Of course, this is a simplified schema. In real life you will have many
additional tables, because there will be several marks per enrollment, and
perhaps several instances of each course, and so on).

So, if Mark and Merry study biology 101, Moses and same Mark study chem
102, and Molly studies computer science 101, you have:

Students:
ID Name ....
001 Mark ....
002 Merry ....
003 Moses ....
004 Molly ....

Courses:
ID Name ....
001 biology 101 ....
002 chem 102 ....
003 computer science 101 ....

Enrollment:
StID CourseID ....
001 001
002 001
003 002
001 002
004 003

Now, you ask yourself, how do I get all students who study biology 101? You use

SELECT s.ID, s.Name
FROM Students s, Courses c, Enrollment e
WHERE c.Name = 'biology 101'
AND c.ID = e.CourseID
AND s.ID = e.StID;

In the same manner, if you want to know all the courses to which Mark is
enrolled, you use

SELECT c.ID, c.Name
FROM Students s, Courses c, Enrollment e
WHERE s.Name = 'Mark'
AND c.ID = e.CourseID
AND s.ID = e.StID;

One last note: for this to be efficient, you have to have indices on the
proper fields. These will always be the primary keys in the two main tables
(the course ID and the student ID), and the corresponding foreign keys in
the relation table (StID and CourseID in enrollment). Additional indices
may be needed if you intend to search by any other field. For example, if
you intend to select by student name, as per the second example query
above, you need to have an index on the Name field in Students as well.

Herouth

--
Herouth Maoz, Internet developer.
Open University of Israel - Telem project
http://telem.openu.ac.il/~herutma