The second week in DAT601 and I must admit I am learning a lot and really enjoying it! at the moment I am reading through this really good power point presentation and entity relationship diagram we have been supplied with in class.
Entity! – what are they?
- A real-world item
- an object with a physical existence
- an object with a conceptual existence ( A Company)
Entity Type?
- An entity type defines a collection of entities that have the same attribute!
- they are defined by its name and attribute
- an instance is the single occurrence of an entity type
Entity Set?
- this is a collection of entities – of the same type
- An entity set is referred to by the same name and the entity type
- An entity type is represented as a rectangle – the name is a singular noun
So what does this all mean? well to me I think it means that is an object that we want to store information about and that they are usually recognizable and they are either, persons, places and even events that relevance to the database!
Attributes describe the properties and are represented by ovals and are then connected to the entity by a single line, for instance, a student would have the following attributes.
- Student ID
- First Name
- Last Name
- Email Address
- Mobile Number
I will go into more detail in my next blog once I have gone over the different types of attributes ( Composite, Single-Valued, Multi-Valued, Derived and Stored) and I’ll also talk more about Keys! just have a bit more reading to do!
NMIT - IT 