Hello!

I'm having trouble distinguishing between an entity and an entity type.

The textbook says that an entity is a "distinct object that is to be represented in the database". The examples of entities it gives are:

• Branch
• Staff
• PropertyForRent
• Client
• PrivateOwner
• Lease

The textbook also defines an entity type as "a group of objects in the real world with the same properties", then goes on to say that we can give "only a working definition of an entity type" listing examples as:

• Staff
• PropertyForRent

Those both have the same examples! And they both seem to be represented the same way (rectangles) in ER diagrams, so how do they differ? Is an entity a single instance of an entity type, or is that an entity occurrence?

Thank you!

As you said:An entity type is a broader classification that groups similar entities together, it contains structure and properties. An entity, we can see it as a single instance of entity type. In ER diagrams: we can use the rectangle labeled (e.g:"Staff") represents the entity type. If we want to insert some values into the db,we can give a staff member, liken "Tom".

Hi Harrison,

'Entity: Entity is a concept to abstractly represent all instances of a group of similar “things”.

Entity Type: It is collection of entity having common attribute.

Entity Instance: An entity instance is a single occurrence of an entity.'

from https://stackoverflow.com/questions/54722484/whats-the-difference-between-entity-entity-type-and-entity-instance#:~:text=Entity%3A%20Entity%20is%20a%20concept,single%20occurrence%20of%20an%20entity.

At this stage everything is still conceptual, so you're trying to identify all of the concepts that need to be stored in the database, we also have specialisations and generalisations of entities, and enhanced ER modelling too, so you have to look at it all a little more abstractly.

Thanks, Adam.

"Harrison Lisle" [email protected] wrote:

Hello!

I'm having trouble distinguishing between an entity and an entity type.

The textbook says that an entity is a "distinct object that is to be represented in the database". The examples of entities it gives are:

• Branch
• Staff
• PropertyForRent
• Client
• PrivateOwner
• Lease

The textbook also defines an entity type as "a group of objects in the real world with the same properties", then goes on to say that we can give "only a working definition of an entity type" listing examples as:

• Staff
• PropertyForRent

Those both have the same examples! And they both seem to be represented the same way (rectangles) in ER diagrams, so how do they differ? Is an entity a single instance of an entity type, or is that an entity occurrence?

Thank you!