Page 92 - DCAP402_DCAO204_DATABASE MANAGEMENT SYSTEM_MANAGING DATABASE

Unit 6: Relational Language and Database Design

Objectives Notes

After studying this unit, you will be able to:
 Discuss relational calculus, tuple and domain relational calculus

 Explain entity relationship model
 Know the constraints
 Describe E-R diagrams and extended ER features

Introduction

Relational tables can be considered as sets. The rows of the tables can be considered as elements
of the set. Operations that can be performed on sets can be done on relational tables. Relational
Data operators are used to retrieve the required data from relational tables. Data is retrieved
using queries and the queries are formulated using various data operators.
To be able to use a database to store data, it should be designed in an efficient manner. The first
step in designing a database is data modeling. Data modeling enables a database designer to
create a model that represents the way in which information is likely to be organized in the
database. There are two major methodologies used to create a data model: the Entity-Relationship
(ER) approach and the Object Model. This unit mainly focuses on data modeling using the
Entity-Relationship approach. The basic techniques described here are applicable to the
development of relational database applications.

6.1 Relational Calculus

Relational calculus is an alternative to relational algebra. In contrast to the algebra, which is
procedural, the calculus is nonprocedural, or declarative, in that it allows us to describe the set
of answers without being explicit about how they should be computed. Relational calculus has
had a bid influence on the design of commercial query languages such as SQL and, especially,
Query-by-Example (QBE).

The variant of the calculus that we present in detail is called the tuple relational calculus (TRC),
variables in TRC take on tuples as values. In another variant, called the domain relational
calculus (DRC), the variables range over field values. TRC has had more of an influence on SQL,
while DRC has strongly influenced QBE.
6.2 Tuple Relational Calculus

A tuple variable is a variable that takes on tuples of a particular relation schema as values. That
is, every value assigned to a given tuple variable has the same number and type of _elds. A tuple
relational calculus query has the form {T | p (T)}, where T is a tuple variable and p(T) denotes a
formula that describes T; we will shortly define formulas and queries rigorously. The result of
this query is the set of all tuples t for which the formula p(T) evaluates to true with T = t. The
language for writing formulas p(T) is thus at the heart of TRC and is essentially a simple subset
of first-order logic. As a simple example, consider the following query.

Example: Find all sailors with a rating above 7.
{ S | S Sailors S. rating > 7}

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