| Entity And Entity Set |
- 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 >domainof 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)}.
An analogy can be made with the programming language notion of type definition.
- 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 anentity in
the E-R model.
Figure 2-1 shows two entity sets.
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 arecustomer-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 areaccount-number and balance.
- transaction>, the set of all account transactions executed in
the bank. Attributes are transaction-number, date and
amount
|
