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Lesson 3 Relational theory
Objective ER diagram as relational notation.
In the preceding course Relational Database Design, you created a Entity Relationship diagram to track
  1. the products offered by and
  2. orders placed with Stories on CD, Inc.,
a fictional company that sells books on CD principally by mail order.
In this module you will use a modified version of that ER diagram to derive the database objects used to represent CDs, distributors, and orders in the database.
When you begin to design the structure of your database, you will represent entities in the database as relations.
Relations are expressed in a shorthand called relational notation, a textual interpretation of the ER diagram. For example, a relation to store data about items stocked by Stories on CD, Inc. might be expressed this way in relational notation:
  1. ER diagram: (Entered in the glossary db under entity-relationship diagram.)
    A diagram used during the design phase of database development to illustrate the organization of and relationships between data during database design.
  2. Relational notation: Relations are expressed in a shorthand called relational notation, a textual interpretation of the ER diagram.
ERD Notation ERD Notation Mouse Out
Instructions
CD Column Entity
Relational Notation Explained
Note the similarities between this relation and the CD entity in the project ER diagram.
CD entity
Other elements of relational constructs include the data domain of each field, the primary key of each relation, and any foreign keys. You will learn about each of those elements later in this module.
Translating an ER diagram into relational notation is a common step in the logical design stage of the database life cycle.
Once you have created relational notations based on the entities in your ER diagram, you can analyze them with an eye toward:
  1. Ensuring that information is delivered in a consistent manner
  2. Eliminating data redundancy
  3. Preserving existing data when deleting unwanted data
It is possible to perform the analysis in this module and the next with the entities written as they are in the ER diagram, but representing the entities in relational notation makes the task easier in three important ways:
  1. Representing an entity in a text format is much more compact than representing the same entity graphically.
  2. Some of the concepts in this course, such as functional dependencies and transitive dependencies, are customarily written (and are easier to explain) using relational notation.
  3. Finally, if you choose to continue studying database design, you will almost certainly encounter relational notation in other courses or books.
Relational Database Notation
Being able to read and write relational notation is a valuable skill.
The next lesson outlines the characteristics of tables, which are the objects that store database data.