Entity Relationship Types

• What is an Entity? #1

  • An entity in something worthy of capturing and storing data about e.g. students, orders, products, courses, customers 
  • Entities become tables in a relational database 
  • Relational databases store different entities in separate tables. Linking tables depends on the relationships between entities.
• What is an entity #2

  There are 3 types of (sometimes called degrees of) relationships: 

  • One to one 
  • One to many 
  • Many to many 
• What is an entity #3

  Imagine a company has  

  • A table of products 
  • A table of customers 
  • A table of the orders the customers have made 

  What is the relationship between a customer and an order?

  • One customer can make multiple (many) orders 
  • But each order relates to a specific (one) customer 
  • So the relationship between customer and order is one to many 
• What is an entity#4

  Now consider the relationship between a product and an order 

  • An order could have more than one (many) products on it 
  • A product could be on more than one (many) order 
  • So the relationship between order and product is many to many 

  One to one relationships also exist but are not very common in databases 
• Drawing Entity Relationship Diagrams 

  • Entity Relationship Diagrams (ERDs) are simple diagrams that represent the entities (tables) that will be in a database and the relationships between these entities 
  • The entities are drawn as boxes with the entity name in 
  • The relationships are drawn in as what is known as ‘crow’s feet notation’ 

  
• Resolving Many to Many Relationships 

  • Many to many relationships cannot be implemented into an actual database and need to be ‘resolved’ 
  • This involves creating a new table known as a ‘link’ table that goes between the entities 
  • This new table links the entities that have the many to many relationships together