Boyce–Codd normal form

Boyce–Codd normal form (or BCNF or 3.5NF) is a normal form used in database normalization. It is a slightly stronger version of the third normal form (3NF). A table is in Boyce–Codd normal form if and only if for every one of its nontrivial dependencies X → Y, X is a superkey—that is, X is either a candidate key or a superset thereof.

BCNF was developed in 1974 by Raymond F. Boyce and Edgar F. Codd to address certain types of anomaly not dealt with by 3NF as originally defined.[1]

Chris Date has pointed out that a definition of what we now know as BCNF appeared in a paper by Ian Heath in 1971.[2] Date writes:

"Since that definition predated Boyce and Codd's own definition by some three years, it seems to me that BCNF ought by rights to be called Heath normal form. But it isn't."[3]

Contents

3NF tables not meeting BCNF

Only in rare cases does a 3NF table not meet the requirements of BCNF. A 3NF table which does not have multiple overlapping candidate keys is guaranteed to be in BCNF.[4] Depending on what its functional dependencies are, a 3NF table with two or more overlapping candidate keys may or may not be in BCNF

An example of a 3NF table that does not meet BCNF is:

Today's Court Bookings
Court Start Time End Time Rate Type
1 09:30 10:30 SAVER
1 11:00 12:00 SAVER
1 14:00 15:30 STANDARD
2 10:00 11:30 PREMIUM-B
2 11:30 13:30 PREMIUM-B
2 15:00 16:30 PREMIUM-A
  • SAVER, for Court 1 bookings made by members
  • STANDARD, for Court 1 bookings made by non-members
  • PREMIUM-A, for Court 2 bookings made by members
  • PREMIUM-B, for Court 2 bookings made by non-members

The table's superkeys are:

Note that even though in the above table Start Time and End Time attributes have no duplicate values for each of them, we still have to admit that in some other days two different bookings on court 1 and court 2 could start at the same time or end at the same time. This is the reason why {Start Time} and {End Time} cannot be considered as the table's superkeys.

However, only S1, S2, S3 and S4 are candidate keys (that is, minimal superkeys for that relation) because e.g. S1 ⊂ S5, so S5 cannot be a candidate key.

Recall that 2NF prohibits partial functional dependencies of non-prime attributes (i.e. an attribute that does not occur in ANY candidate key) on candidate keys, and that 3NF prohibits transitive functional dependencies of non-prime attributes on candidate keys.

In Today's Court Bookings table, there are no non-prime attributes: that is, all attributes belong to some candidate key. Therefore the table adheres to both 2NF and 3NF.

The table does not adhere to BCNF. This is because of the dependency Rate Type → Court, in which the determining attribute (Rate Type) is neither a candidate key nor a superset of a candidate key.

Dependency Rate Type → Court is respected as a Rate Type should only ever apply to a single Court.

The design can be amended so that it meets BCNF:

Rate Types
Rate Type Court Member Flag
SAVER 1 Yes
STANDARD 1 No
PREMIUM-A 2 Yes
PREMIUM-B 2 No
Today's Bookings
Rate Type Start Time End Time
SAVER 09:30 10:30
SAVER 11:00 12:00
STANDARD 14:00 15:30
PREMIUM-B 10:00 11:30
PREMIUM-B 11:30 13:30
PREMIUM-A 15:00 16:30


The candidate keys for the Rate Types table are {Rate Type} and {Court, Member Flag}; the candidate keys for the Today's Bookings table are {Rate Type, Start Time} and {Rate Type, End Time}. Both tables are in BCNF. Having one Rate Type associated with two different Courts is now impossible, so the anomaly affecting the original table has been eliminated.

Achievability of BCNF

In some cases, a non-BCNF table cannot be decomposed into tables that satisfy BCNF and preserve the dependencies that held in the original table. Beeri and Bernstein showed in 1979 that, for example, a set of functional dependencies {AB → C, C → B} cannot be represented by a BCNF schema.[5] Thus, unlike the first three normal forms, BCNF is not always achievable.

Consider the following non-BCNF table whose functional dependencies follow the {AB → C, C → B} pattern:

Nearest Shops
Person Shop Type Nearest Shop
Davidson Optician Eagle Eye
Davidson Hairdresser Snippets
Wright Bookshop Merlin Books
Fuller Bakery Doughy's
Fuller Hairdresser Sweeney Todd's
Fuller Optician Eagle Eye

For each Person / Shop Type combination, the table tells us which shop of this type is geographically nearest to the person's home. We assume for simplicity that a single shop cannot be of more than one type.

The candidate keys of the table are:

Because all three attributes are prime attributes (i.e. belong to candidate keys), the table is in 3NF. The table is not in BCNF, however, as the Shop Type attribute is functionally dependent on a non-superkey: Nearest Shop.

The violation of BCNF means that the table is subject to anomalies. For example, Eagle Eye might have its Shop Type changed to "Optometrist" on its "Fuller" record while retaining the Shop Type "Optician" on its "Davidson" record. This would imply contradictory answers to the question: "What is Eagle Eye's Shop Type?" Holding each shop's Shop Type only once would seem preferable, as doing so would prevent such anomalies from occurring:

Shop Near Person
Person Shop
Davidson Eagle Eye
Davidson Snippets
Wright Merlin Books
Fuller Doughy's
Fuller Sweeney Todd's
Fuller Eagle Eye
Shop
Shop Shop Type
Eagle Eye Optician
Snippets Hairdresser
Merlin Books Bookshop
Doughy's Bakery
Sweeney Todd's Hairdresser


In this revised design, the "Shop Near Person" table has a candidate key of {Person, Shop}, and the "Shop" table has a candidate key of {Shop}. Unfortunately, although this design adheres to BCNF, it is unacceptable on different grounds: it allows us to record multiple shops of the same type against the same person. In other words, its candidate keys do not guarantee that the functional dependency {Person, Shop Type} → {Shop} will be respected.

A design that eliminates all of these anomalies (but does not conform to BCNF) is possible.[6] This design consists of the original "Nearest Shops" table supplemented by the "Shop" table described above.

Nearest Shops
Person Shop Type Nearest Shop
Davidson Optician Eagle Eye
Davidson Hairdresser Snippets
Wright Bookshop Merlin Books
Fuller Bakery Doughy's
Fuller Hairdresser Sweeney Todd's
Fuller Optician Eagle Eye
Shop
Shop Shop Type
Eagle Eye Optician
Snippets Hairdresser
Merlin Books Bookshop
Doughy's Bakery
Sweeney Todd's Hairdresser


If a referential integrity constraint is defined to the effect that {Shop Type, Nearest Shop} from the first table must refer to a {Shop Type, Shop} from the second table, then the data anomalies described previously are prevented.

References

  1. ^ Codd, E. F. "Recent Investigations into Relational Data Base Systems." IBM Research Report RJ1385 (April 23rd, 1974). Republished in Proc. 1974 Congress (Stockholm, Sweden, 1974). New York, N.Y.: North-Holland (1974).
  2. ^ Heath, I. "Unacceptable File Operations in a Relational Database." Proc. 1971 ACM SIGFIDET Workshop on Data Description, Access, and Control, San Diego, Calif. (November 11th–12th, 1971).
  3. ^ Date, C.J. Database in Depth: Relational Theory for Practitioners. O'Reilly (2005), p. 142.
  4. ^ Vincent, M.W. and B. Srinivasan. "A Note on Relation Schemes Which Are in 3NF But Not in BCNF." Information Processing Letters 48(6), 1993, pp. 281–83.
  5. ^ Beeri, Catriel and Bernstein, Philip A. "Computational problems related to the design of normal form relational schemas." ACM Transactions on Database Systems 4(1), March 1979, p. 50.
  6. ^ Zaniolo, Carlo. "A New Normal Form for the Design of Relational Database Schemata." ACM Transactions on Database Systems 7(3), September 1982, pp. 493.

Bibliography

  • Date, C. J. (1999). An Introduction to Database Systems (8th ed.). Addison-Wesley Longman. ISBN 0-321-19784-4.

External links