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). BCNF was developed in 1974 by Raymond F. Boyce and Edgar F. Codd to address certain types of anomalies not dealt with by 3NF as originally defined.
If a relational schema is in BCNF then all redundancy based on functional dependency has been removed, although other types of redundancy may still exist. A relational schema R is in Boyce–Codd normal form if and only if for every one of its dependencies X → Y, at least one of the following conditions hold: X → Y is a trivial functional dependency (Y ⊆ X), X is a superkey for schema R. Note that if a relational schema is in BCNF, then it is in 3NF.
3NF table always meeting BCNF (Boyce–Codd normal form)
Only in rare cases does a 3NF table not meet the requirements of BCNF. A 3NF table that does not have multiple overlapping candidate keys is guaranteed to be in BCNF. 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:
Each row in the table represents a court booking at a tennis club. That club has one hard court (Court 1) and one grass court (Court 2) A booking is defined by its Court and the period for which the Court is reserved Additionally, each booking has a Rate Type associated with it. There are four distinct rate types: 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: S1 = { Court, Start time} S2 = { Court, End time} S3 = { Rate type, Start time} S4 = { Rate type, End time} S5 = { Court, Start time, End time} S6 = { Rate type, Start time, End time} S7 = { Court, Rate type, Start time} S8 = { Court, Rate type, End time} ST = { Court, Rate type, Start time, End time}, the trivial superkey
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. See "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 – on which Court depends – (1) is neither a candidate key nor a superset of a candidate key and (2) Court is no subset of Rate type.
Dependency Rate type → Court is respected, since a Rate type should only ever apply to a single Court.
The design can be amended so that it meets BCNF:
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 { Court, Start time} and { Court, End time}. Both tables are in BCNF. When { Rate type} is a key in the Rate types table, having one Rate type associated with two different Courts is impossible, so by using { Rate type} as a key in the Rate types table, 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.
Consider the following non-BCNF table whose functional dependencies follow the { AB → C, C → B} pattern:
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: { Person, Shop type}, { Person, Nearest shop}.