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Boyce-Codd Normal Form (BCNF) BCNF does the following. . A table must be in 3NF. . A table can have only one candidate key. A candidate key has potential for being a table s primary key. A table is not allowed more than one primary key because referential integrity requires it as such. It would be impossible to check foreign keys against more than one primary key. Referential integrity would be automatically invalid, unenforceable, and, thus, there would be no relational database model. BCNF is an odd one because it is a little like a special case of 3NF. BCNF requires that every determinant in a table is a candidate key. If there is only one candidate key, 3NF and BCNF are the same. What does this mean? A determinant is a field whose value may depend other fields for their values. And some tables can be uniquely identified by more than one field or combination of fields. Each one of these is a potential primary key that can uniquely identify records in a table, and, thus, they are candidate keys, or potential primary keys. This is where BCNF gets interesting because most examples of BCNF (and, in fact, any level of normalization beyond 3NF) utilize generally highly complex, multiple table, multiple many-to-many join resolution issues. You will get to those nasty, horribly complicated examples just not yet. The left side of the diagram in Figure 4-33 shows an example of a table with both a surrogate key and a natural key. Surrogate keys are added to tables to replace natural keys because surrogate keys are more efficient and easier to manage than natural keys. The other problem is that the original primary key (the natural key) needs a unique qualifier on it to ensure that it is not duplicated. Two customers with the same name could cause a problem when it comes to sending them both an invoice at the end of the month. The right side of the diagram in Figure 4-33 shows a BCNF breakdown into an innumerate multiple of tables created from the original table, for any values that are potentially unique. This is what the purest form of BCNF essentially requires. It is a little bit ridiculous. 108 Chapter 4
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