Entities and Entity Sets

• An entity is an object that exists and is distinguishable from other objects. For instance, John Harris with S.I.N. 890-12-3456 is an entity, as he can be uniquely identified as one particular person in the universe.

• An entity may be concrete (a person or a book, for example) or abstract (like a holiday or a concept).

• An entity set is a set of entities of the same type (e.g., all persons having an account at a bank).

• Entity sets need not be disjoint. For example, the entity set employee (all employees of a bank) and the entity set customer (all customers of the bank) may have members in common.

• An entity is represented by a set of attributes.
  • E.g. name, S.I.N., street, city for ``customer'' entity.
  • The domain of the attribute is the set of permitted values (e.g. the telephone number must be seven positive integers).

• Formally, an attribute is a function which maps an entity set into a domain.
  • Every entity is described by a set of (attribute, data value) pairs.
  • There is one pair for each attribute of the entity set.
  • E.g. a particular customer entity is described by the set {(name, Harris), (S.I.N., 890-123-456), (street, North), (city, Georgetown)}.

• The concept of an entity set corresponds to the programming language type definition.
• A variable of a given type has a particular value at a point in time.

• Thus, a programming language variable corresponds to an entity in the E-R model.

We will be dealing with five entity sets in this section:

• branch, the set of all branches of a particular bank. Each branch is described by the attributes branch-name, branch-city and assets.

• customer, the set of all people having an account at the bank. Attributes are customer-name, S.I.N., street and customer-city.

• employee, with attributes employee-name and phone-number.

• account, the set of all accounts created and maintained in the bank. Attributes are account-number and balance.

• transaction, the set of all account transactions executed in the bank. Attributes are transaction-number, date and amount.