About Entity Relationship Diagrams
A one-to-one relationship is when one record in one table is associated with only one record in another table.
One-to-one relationship graph looks like this:
A one-to-many relationship is when one record in one table is associated with one or more records in another table.
For example each customer can have multiple sales orders
Many-to-many relationships are when multiple records in one table are associated with multiple records in another table.
For example customers can purchase many products and products can be purchased by many customers.
Drawing Entity Relationship Diagrams
An object in a data repository
Data Entity Relationship Diagram
Unit of information inside a data entity