Entity Relationship Diagrams

A strong entity is one whose existence does not depend on the existence of any other entity in the same database. The primary key of a strong entity uniquely identifies each occurrence within the entity.  

A weak entity is one that depends on one or more strong entities for its existence. For this reason, strong entities are sometimes referred to as owner entities. A weak entity cannot be used independently because its existence depends on one or more owner entities. The primary key of a weak entity is formed, in part, using the primary key of its owner entity(ies). 

If we look at the Supplier, Item, Order and Customer entities a record can be uniquely identified using their own attributes. So these would be classed as strong entities. The OrderItem entities’ Primary Key depends on attributes from other entities so it would be classed as weak entity.

A weak entity always depends on the strong entity for its existence whereas, a strong entity is independent of any other entity’s existence. So when classifying an entity as strong or weak the relationship between the entities is key.

If we look at the supplier and item entities we will see that every item has a supplier but not every supplier has an item.

If we look at the item and orderitem entities we will see that every orderitem is an item but not every item is an orderitem.

If we look at the order and orderitem entities we will see that every orderitem is an order but not every order has every item.

If we look at the order and customer entities we will see that every order is made by a customer but not every customer has placed an order.

A travel agency uses a relational database to store details on a booking system.

It stores details of Scottish holiday resorts, hotels in each resort, customers and their bookings. These details are arranged in four separate entities as shown below.

From the list of attributes, we can see that Resort, Hotel and Customer are all strong entities while Booking is a weak entity.

An entity-occurrence diagram indicating the relationships between the entities is shown below. Using an entity-occurrence diagram helps to clarify the nature of each relationship.

From the entity occurrence diagram we can see that not every resort has a hotel but every hotel is at a resort.

From the entity occurrence diagram we can see that not every hotel as a booking but every booking must have an associated hotel.

From the entity occurrence diagram we can see that not every customer has made a booking but every booking must be by a a customer