9.2 The E-R Model Entities and Attributes

	An entity is the basic object in an E-R model.
	It represents an object in the real world that can be distinctly identified.
	For example, library database can include entities such as book, member, department, etc.
	Two types of entities can be specified in the E-R model which are:

Weak entity: that is its existence is dependent on another entity.

			For example, a borrowedBook entity could be a weak entity as its existence depends on the member entity.
			If a library member is deleted, all the borrowed books records in borrowedBook must be deleted too.
			The member entity is called the identifying owner.
			The relationship (see Lecture 10) between the weak entity and the identifying owner entity is called the identifying relationship of the weak entity.

Regular entity: its existence is not dependent on any other entity.

	Graphically, in an E-R model, a regular entity is represented by a rectangle box.
	Weak entities are distinguished with a double box notation as shown in Figure 9.1.

9.2 The E-R Model Entities and Attributes

	An entity can have a number of attributes which represent the properties of the entity.
	E.g., all books entities can have attributes such as book title, book author and publisher.
	Each attribute is associated with a domain which specifies the type the attribute’ s values.
	Attributes in an E-R model can be:

Composite or Simple attributes:

			A composite attribute can be composed of several simple attributes.
			For example, the Member Name attribute can be a composite attribute which can contain attributes First Name, Middle Initials and Last Name.

9.2 The E-R Model Entities and Attributes

			Note that a composite attribute can have another composite attribute as part of it.
			A simple attribute is not divisible, also called an atomic attribute. First Name, Middle Name and Last Name can be simple attributes.

Key Attribute:

			The value of a key attribute of an entity instance uniquely identifies the entity instance.
			For example, a Member ID attribute can be the key attribute in a Member entity as no two members can have the same member id.
			A key can be formed of several attributes.

Single-valued or Multi-valued Attributes:

			Most attributes are single-valued, i.e. can have at most one value.
			For example, a First Name attribute in a Member entity can have one value only, e.g. “smith”.
			However, attributes can also be multi-valued.
			An example could be a Qualification attribute in a Author entity, which can hold more than one value representing different qualifications for an author.

9.2 The E-R Model Entities and Attributes

Base or Derived Attributes:

			Attributes can be related in a way that one attribute is derived from another attribute.
			An example is a Borrowing Duration attribute which can be derived from the Date of Borrowing and Date of Return of a book.

Missing Attribute:

			When there is no applicable value for an attribute, a special value Null is used.
			For example, a Qualification attribute may have value Null when an author has no qualification.

9.2 The E-R Model Entities and Attributes