Relational Constructs   «Prev

# Relational Theory Overview

Question: Is representing entities with relational notation easier to understand than representing the same entities graphically?
The answer is that for some students relational notation will seem quite natural and for others it will take some getting used to. I will continue to represent entities using both styles, so if you prefer graphics to relational notation you will have a visual reference to make the explanations more understandable. All I ask is that you learn how to read relational notation. Doing so will allow you to study beyond this course and take advantage of resources that use relational notation to present their material.
1. Domain: Determines the type of data values that are permitted for that attribute.
2. Primary key: A field (or combination of fields) that uniquely identifies a record in a table.
3. Foreign key: A field (or combination of fields) used to link tables; a corresponding primary key field occurs in the same database.
4. Data redundancy: Duplication of data in a database.

Relational databases play a critical role in many important computer applications. As is the case whenever enormous amounts of money are at stake, people have spent a huge amount of time and effort building, studying, and refining relational databases. Database researchers usually approach relational databases from one of three points of view.

## First Group

The first group approaches the problem from a database-theoretical point of view. These people tend to think in terms of provability, mathematical set theory, and propositional logic.
Theorists use phrases such as
1. relational algebra,
2. Cartesian product, and
3. tuple relational calculus.
This approach is intellectually stimulating and can be a bit intimidating. These researchers focus on logical design and idealized database principles.

## Second Group

The second group approaches the topic from a less formal and practical point of view. Their terminology tends to be less precise and rigorous but more intuitive. They tend to use terms that you may have heard before such as table, row, and column. These people focus on physical database design and pay more attention to concrete bits-and-bytes issues dealing with actually building a database and getting the most out of it.

### Third Group

The third group tends to think in terms of flat files and the underlying disk structure used to hold data. Though these people are probably in the minority these days, their terms file, record, and field made their way into database nomenclature and have remained. Many of those who still use these terms are programmers and other developers who look at the database from a consumer's " How do I get my data point of view". These differing points of view have led to several different and potentially confusing ways to view relational databases. This can cause some serious confusion, particularly because the different groups have latched on to some of the same terms but used for different meanings. In fact, they sometimes use the term relation in very different ways .