Entity Set

Requirements for Weak Entity Sets

We cannot get key attributes for a weak entity set indiscriminately. Rather, if E is a weak entity set then its key comprises:

Weak Entity Set Notation

We shall adopt the following principles to show that an entity set is weak and to declare its key attributes.

Attributes / Schemas

Across the top of a relation we see attributes; in the following figure, the attributes are title, year, length, and filmType. Attributes of a relation serve as names for the columns of the

From Entity Sets to Relations

We shall first consider entity sets that are not weak. We shall take up the modifications required to accommodate weak entity sets in "Handling Weak Entity Sets". For each

Combining Relations

Often, the relations that we get from converting entity sets and relationships to relations are not the best possible choice of relations for the given data. One ordinary situation occurs when there

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

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

The Transitive Rule

The transitive rule lets us cascade two FD's. ● If A1A2 . . . An → B1B2 . . . Bm and B1B2 . . . Bm → C1C2 . . . Ck hold in relation R, then A1A2 . . . An → C1C2 . . . Ck also holds in R.

The Type System

An object-oriented programming language offers the user a rich collection of types. Starting with atomic types, such as integers, real numbers, booleans, and character strings, one may construct new types by using type constructors. Usually, the type constructors let us

Extents

When an ODL class is part of the database being described, we need to differentiate the class definition itself from the set of objects of that class that exist at a given time. The difference is the same as that between a relation schema and a relation instance, even though both can

Page 3 of 4 Previous 1 2 3 4 Next