This reading covers the Entity Relationship Diagram (ER-D) and demonstrates how to use it to support the design of a relational database.
The relational database model organizes information into tables to ensure a good data structure to maintain consistency and accuracy, which makes the design of the tables and their relationships very crucial. The relational database design is very well connected with the entity relationship modelling process including entities, attributes and relationship identification and definition. The entity-relationship diagram (ER-D) is commonly used to represent and document the entity relationship models.
The use of entity relationship diagrams helps to provide the big picture of your database. It also ensures the data requirements and operations are well defined and documented in your project. In addition, the ER-D represents a blueprint that guides database developers through the implementation of the actual database in a relevant database management system such as Oracle and MySQL.
The entities, attributes and relationships between entities can be shown in a variety of diagrammatic formats in the ER diagrams. In this reading, you’ll review the most used shapes and symbols.
In the ER-D, a box with two compartments is used to represent the entity and its related attributes. The top compartment represents the entity name, and the bottom compartment includes the related attributes.
For example, a college enrollment system contains a database with information about the students, and the courses available in each department.
In this case, you can have three entities represented in three separate boxes:
There’s no point in considering entities or attributes that will not be used in your project. You should only capture data that helps the users of your database system to complete certain tasks and activities.
Relationship representation
The ER diagram uses different styles of lines to define the distinct types of relationships between entities. The line style depends on the cardinality of the relationship, which refers to the number of elements in a set of data as clarified in the following three cases.
1:1 (one-to-one): The ER-D uses a straight-line representation for a one-to-one cardinality relationship. For example, each passenger on a train should have only one ticket.
1:N (one-to-many): The ER-D is a straight line with a crow’s foot notation on one side only to represent a one-to-many cardinality relationship. For example, one parent can have many children.
