Lesson 7- Entity Relational Diagrams
The entity relationship model is a drawing technique used to analyse the nature of an applications data. The product of this model is an Entity Relationship Diagram.
Entity Relationship Diagrams (ERDs)
Note: As with many things in database design you may see many different
notations for these, depending on the toolset used to produce them. On a first
level diagram we will use a simple box to represent an entity.
The relationships between entities is shown by a connecting line labelled with a relationship name.
Note that the relationship can be read in either direction
• one school managed by one head
• one head manages one school
Note also that the entity name is always singular.
There are several types of relationship,
• a book has one publisher
(one to one relationship)
• but may have many authors (a one to many
relationship)
This part of the relationship is referred to as its Cardinality
In an ERD diagram the many end of the relationship is shown as a splayed line commonly called a crowsfoot.
Participation of an entity in a relationship may be mandatory or optional, the solid line indicates mandatory participation the dotted optional.
The ERD is a top level (overview) design document which is then refined into a database design.
