Instances of an E/R Diagram

E/R diagrams provide details for describing the schema of databases, that is, their structure. A database described by an E/R diagram will include specific data, which we call the database instance. Particularly, for each entity set, the database instance will have a particular finite

From ODL Designs to Relational Designs

As the E/R model is meant to be converted into a model such as the relational model when we put into operation the design as an actual database, ODL was initially meant to be used as the specification language for real, object-oriented DBMS's. Nevertheless ODL, like all object-

Representing Set-Valued Attributes

Record structures are not the most complicated kind of attribute that can appear in ODL class definitions. Values can also be made using type constructors Set, Bag, List, Array, and Dictionary from Types in ODL. Each presents its own problems when migrating to the relational


The fact that movies like Star Wars will appear in various relations that are values of the movies attribute in the nested relation Stars is a cause of redundancy. In effect, the schema of Nested Relations Example (a) has the nested-relation analog of not being in BCNF. On the

Object-Oriented Versus Object-Relational

The object-oriented data model, as typified by ODL, and the object-relational model discussed here, are outstandingly similar. Some of the most important points of comparison follow.

Natural Joins / Theta-Joins

More frequently than we want to take the product of two relations, we find a need to join them by pairing only those tuples that match in some way. The simplest kind of match is the natural join of two relations R and S, denoted R x S, in which we pair only those tuples from R

Constraints on Relations

Relational algebra offers a means to express common constraints, such as the referential integrity constraints introduced in The Modeling of Constraints. Actually, we shall see that relational algebra provides us convenient ways to express a large variety of other constraints. Even

Referential Integrity Constraints

A common kind of constraint, called "referential integrity" in The Modeling of Constraints, declares that a value appearing in one context also appears in another, related context. We saw referential integrity as a matter of relationships "making sense". That is, if an object or

The Truth-Value UNKNOWN

In Selection in SQL we assumed that the result of a comparison was either TRUE or FALSE, and these truth-values were combined in the obvious way using the logical operators AND, OR and NOT. We have just seen that when NULL values occur, comparisons can yield a third


In SQL, one query can be used in many ways to help in the evaluation of another. A query that is part of another is called a subquery. Subqueries can have subqueries, and so on, down as many levels as we desire. We already saw one example of the use of subqueries; in

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