Databases Reference
In-Depth Information
Chapter 4
F D s a n d B C N F
( I n f o r m a l )
It is downright sinful to teach the abstract before the concrete
—Z. A. Melzak: Companion to Concrete Mathematics
As we saw in the previous chapter, Boyce/Codd normal form (BCNF for short) is defined in terms of functional
dependencies. In fact, BCNF is really the normal form with respect to functional dependencies (just as—to get
ahead of ourselves for a moment—5NF is really the normal form with respect to join dependencies). The overall
purpose of the present chapter is to explain this observation; as the chapter title indicates, however, the various
explanations and associated definitions are all (intentionally, of course) a little informal at this stage. (Informal, but
not inaccurate; I won't tell any deliberate lies.) A more formal treatment of the material appears in the next chapter.
FIRST NORMAL FORM
To begin at the beginning: Let relation r have attributes A1 , ..., An , of types T1 , ..., Tn , respectively. By definition,
then, if tuple t appears in relation r , then the value of attribute Ai in t is of type Ti ( i = 1, ..., n ). For example, if r is
the current value of the shipments relvar SP (see Fig. 1.1 in Chapter 1), then every tuple in r has an SNO value that's
of type CHAR, a PNO value that's also of type CHAR, and a QTY value that's of type INTEGER.
Now I can give a precise definition of first normal form: 1
Definition: Let relation r have attributes A1 , ..., An , of types T1 , ..., Tn , respectively. Then r is in first
normal form (1NF) if and only if, for all tuples t appearing in r , the value of attribute Ai in t is of type Ti
( i = 1, ..., n ).
In other words, every relation is in 1NF, by definition! To say it in different words, 1NF just means each
tuple in the relation contains exactly one value, of the appropriate type, for each attribute. Observe in particular that
1NF places no limitations on what those attribute types are allowed to be. 2 They can even be relation types; that is,
relations with relation valued attributes —RVAs for short—are legal (you might be surprised to hear this, but it's
true). An example is given in Fig. 4.1.
1 One reviewer accused me of rewriting history with this definition. Guilty as charged, perhaps─but I do have my reasons; to be specific, earlier
“definitions” of the concept were all, in my opinion, either too vague to be useful or flat out wrong. See SQL and Relational Theory for further
discussion.
2 The relational model does, though. To paraphrase the answer to Exercise 2.2 in Appendix D, there are two small exceptions, both of which I'll
simplify just slightly here: First, no relation in the database can have an attribute of any pointer type; second, if relation r is of type T , then no
attribute of r can itself be of type T (think about it!). However, these exceptions have nothing to do with 1NF as such.
Search WWH ::

Custom Search