The set of all such child-lunch relationships in a database is a relationship set. The eaten relationship between a child and her lunch is a single relationship. The collection of all songs in a database is an entity set. Rather, they show entity sets and relationship sets. Examples: an employee entity might have a Social Security Number (SSN) attribute the proved relationship may have a date attribute.Įvery entity (unless it is a weak entity) must have a minimal set of uniquely identifying attributes, which is called the entity's primary key.Įntity-relationship diagrams don't show single entities or single instances of relations. ERROL's semantics and implementation are based on Reshaped relational algebra (RRA), a relational algebra which is adapted to the ERM and captures its linguistic aspect.Įntities and relationships can both have attributes. The model's linguistic aspect described above is utilized in the declarative database query language ERROL, which mimics natural language constructs. Examples: an owns relationship between a company and a computer, a supervises relationship between an employee and a department, a performs relationship between an artist and a song, a proved relationship between a mathematician and a theorem. Relationships can be thought of as verbs, linking two or more nouns. Examples: a computer, an employee, a song, a mathematical theorem.Ī relationship captures how two or more entities are related to one another. Because the term entity-type is somewhat cumbersome, most people tend to use the term entity as a synonym for this term.Įntities can be thought of as nouns. There are usually many instances of an entity-type. An entity, strictly speaking, is an instance of a given entity-type. Although the term entity is the one most commonly used, following Chen we should really distinguish between an entity and an entity-type. Īn entity may be a physical object such as a house or a car, an event such as a house sale or a car service, or a concept such as a customer transaction or order. When we speak of an entity we normally speak of some aspect of the real world which can be distinguished from other aspects of the real world. An entity is an abstraction from the complexities of some domain. Note that sometimes, both of these phases are referred to as "physical design".Īn entity may be defined as a thing which is recognized as being capable of an independent existence and which can be uniquely identified. In the case of the design of an information system that is based on a database, the conceptual data model is, at a later stage (usually called logical design), mapped to a logical data model, such as the relational model this in turn is mapped to a physical model during physical design. ![]() ![]() an overview and classifications of used terms and their relationships) for a certain area of interest. The data modeling technique can be used to describe any ontology (i.e. The first stage of information system design uses these models during the requirements analysis to describe information needs or the type of information that is to be stored in a database. ![]()
0 Comments
Leave a Reply. |