Attributes

Handling Weak Entity Sets

When a weak entity set appears in an E/R diagram, we need to do three things in different ways. 1. The relation for the weak entity set W itself must contain not only the attributes of W but also the key attributes of the other entity sets that help form the

Converting Subclass Structures to Relations

When we have an isa-hierarchy of entity sets, we are presented with numerous choices of strategy for conversion to relations. Recall we assume that:

An Object-Oriented Approach

Another strategy for converting isa-hierarchies to relations is to enumerate all the possible subtrees of the hierarchy. For each, create one relation that represents entities that have

Using Null Values to Combine Relations - Comparison of Approaches

There is another way to representing information regarding a hierarchy of entity sets. If we are allowed to use NULL (the null value as in SQL) as a value in tuples, we can handle a hierarchy of entity sets with a single relation. This relation has all the attributes belonging to

Keys of Relations

We say a set of one or more attributes {A1, A2,. . . ,An} is a key for a relation R if: 1. Those attributes functionally determine all other attributes of the relation. That is, because relations are sets, it is impossible for two different tuples of R to agree on all of A1, A2, ... ,An.

Rules About Functional Dependencies

In this section, we shall learn how to reason about FD's. That is, suppose we are told of a set of FD’s that a relation satisfies. Sometimes, we can deduce that the relation must satisfy

The Splitting/Combining Rule

Recall that in “Functional Dependencies” we defined the FD: A1A2…An → B1B2..Bm to be a shorthand for the set of FD's:

Trivial Functional Dependencies

A FD A1A2 . . . An → B is said to be trivial if B is one of the A's. For instance, title year → title is a trivial FD. Every trivial FD holds in every relation, since it says that "two tuples that agree in all of A1,

Computing the Closure of Attributes

Before proceeding to other rules, we shall give a general principle from which all rules follow. Assume {A1, A2,. . . ,An} is a set of attributes and S is a set of FD's. The closure of {A1, A2, .

Why the Closure Algorithm Works

In this section, we shall show why the closure algorithm correctly decides whether or not a FD A1A2 . . . An → B follows from a given set of FDs S. There are two parts to the proof:

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