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- Anomalies can occur in relations in 3NF if there is a composite key in which part of that key has a determinant which is not itself a candidate key.
- This can be expressed as R(A,B,C), C--->A where:
- The relation R contains attributes A, B and C.
- A and B form a candidate key.
- C is the determinant for A (A is functionally dependent on C).
- C is not part of any key.
- Anomalies can also occur where a relation contains several candidate keys where:
- The keys contain more than one attribute (they are composite keys).
- An attribute is common to more than one key.
Example to understand BCNF:-
Take the following table:
Note that no two buildings on any of the university campuses have the same name, thus ROOM/BLDG----->CAMPUS. As the determinant is not a candidate key this table is NOT in Boyce-Codd normal form.
This table should be decomposed into the following relations:
R1(course, class, room/bldg, time)