Question

I’ve got one table containing some sort of items, and two tables (a and b) to which these items belong to.

One item refers to either one row in a or one row in b. One a or b can have multiple items.

Is there a better design than the following (using Oracle 10)?

Any pitfalls to watch out for?

id   a_id(fk)    b_id(fk)
 1          1        NULL
 2          1        NULL
 3       NULL           1
 4          2        NULL

Answer 1

That’s a pretty good design on a number of counts:

• With the foreign key on a_id you enforce the link to table A.
• with the foreign key on b_id you enforce the link to table B.
• The 1:many relationships (between items and table A, and between items and table B) are each correctly stored.

The only snag is that this database structure by itself doesn’t check that an item is linked to only one of A or B (and not both). Check constraints on the Items table will do this job.

Pseudo-code example:

CONSTRAINT a_eor_b CHECK 
  (
    NOT (a_id IS NULL AND b_id IS NULL)
    AND NOT (a_id IS NOT NULL AND b_id IS NOT NULL)
  )