The Entity Relationship Model - Learning MySQL [Book]
An entity–relationship model (ER model for short) describes interrelated things of interest in a Diagrams created to represent attributes as well as entities and relationships may tables, database indexes such as unique key indexes, and database constraints such as a foreign key constraint or a commonality constraint . Entity – rectangle; Attribute – oval; Relationship – diamond; Link - line Represented by oval on E-R diagram; Ex: name, maximum enrollment. An entity relationship model, also called an entity-relationship (ER) It's one or more attributes that uniquely identify a weak entity for a given owner entity. In our example, the dependent name is unique for every employee.
Entity—relationships and semantic modeling[ edit ] Semantic model[ edit ] A semantic model is a model of concepts, it is sometimes called a "platform independent model". It is an intensional model. At the latest since Carnapit is well known that: The first part comprises the embedding of a concept in the world of concepts as a whole, i.
The second part establishes the referential meaning of the concept, i. Extension model[ edit ] An extensional model is one that maps to the elements of a particular methodology or technology, and is thus a "platform specific model". The UML specification explicitly states that associations in class models are extensional and this is in fact self-evident by considering the extensive array of additional "adornments" provided by the specification over and above those provided by any of the prior candidate "semantic modelling languages".
It incorporates some of the important semantic information about the real world. Plato himself associates knowledge with the apprehension of unchanging Forms The forms, according to Socrates, are roughly speaking archetypes or abstract representations of the many types of things, and properties and their relationships to one another.
Limitations[ edit ] ER assume information content that can readily be represented in a relational database. They describe only a relational structure for this information.
They are inadequate for systems in which the information cannot readily be represented in relational form[ citation needed ], such as with semi-structured data. For many systems, possible changes to information contained are nontrivial and important enough to warrant explicit specification.
An alternative is to model change separately, using a process modeling technique. Additional techniques can be used for other aspects of systems. For instance, ER models roughly correspond to just 1 of the 14 different modeling techniques offered by UML. Even where it is suitable in principle, ER modeling is rarely used as a separate activity. One reason for this is today's abundance of tools to support diagramming and other design support directly on relational database management systems.
These tools can readily extract database diagrams that are very close to ER diagrams from existing databases, and they provide alternative views on the information contained in such diagrams.
In a survey, Brodie and Liu  could not find a single instance of entity—relationship modeling inside a sample of ten Fortune companies. Badia and Lemire  blame this lack of use on the lack of guidance but also on the lack of benefits, such as lack of support for data integration.
The enhanced entity—relationship model EER modeling introduces several concepts not in ER modeling, but are closely related to object-oriented design, like is-a relationships. For modelling temporal databasesnumerous ER extensions have been considered.
The Entity-Relationship Model
Each product entity would have name, price, and product ID attributes. The ER diagram representation of the product entity Representing Relationships Entities can participate in relationships with other entities. For example, a customer can buy a product, a student can take a course, an artist can record an album, and so on. Like entities, relationships can have attributes: Our database could then record each sale and tell us, for example, that at 3: For example, each customer can buy any number of products, and each product can be bought by any number of customers.
This is known as a many-to-many relationship. We can also have one-to-many relationships. For example, one person can have several credit cards, but each credit card belongs to just one person. Looking at it the other way, a one-to-many relationship becomes a many-to-one relationship; for example, many credit cards belong to a single person.
Finally, the serial number on a car engine is an example of a one-to-one relationship; each engine has just one serial number, and each serial number belongs to just one engine. We often use the shorthand terms 1: N for one-to-one, one-to-many, and many-to-many relationships, respectively. The number of entities on either side of a relationship the cardinality of the relationship define the key constraints of the relationship. There are many relationships that may at first seem to be one-to-one, but turn out to be more complex.
For example, people sometimes change their names; in some applications, such as police databases, this is of particular interest, and so it may be necessary to model a many-to-many relationship between a person entity and a name entity.
Redesigning a database can be time-consuming if you assume a relationship is simpler than it really is. In an ER diagram, we represent a relationship set with a named diamond.
The cardinality of the relationship is often indicated alongside the relationship diamond; this is the style we use in this book.Theory of Attributes - Part 4 - Statistics
The ER diagram representation of the customer and product entities, and the sale relationship between them. Partial and Total Participation Relationships between entities can be optional or compulsory. In our example, we could decide that a person is considered to be a customer only if they have bought a product.
On the other hand, we could say that a customer is a person whom we know about and whom we hope might buy something—that is, we can have people listed as customers in our database who never buy a product. These are referred to as the participation constraints of the relationship.
In an ER diagram, we indicate total participation with a double line between the entity box and the relationship diamond. From time to time, we encounter cases where we wonder whether an item should be an attribute or an entity on its own.
For example, an email address could be modeled as an entity in its own right. When in doubt, consider these rules of thumb: Is the item of direct interest to the database?
Learning MySQL by Hugh E. Williams, Saied M.M. Tahaghoghi
Objects of direct interest should be entities, and information that describes them should be stored in attributes. Our inventory and sales database is really interested in customers, and not their email addresses, so the email address would be best modeled as an attribute of the customer entity. Does the item have components of its own? If so, we must find a way of representing these components; a separate entity might be the best solution.
In the student grades example at the start of the chapter, we stored the course name, year, and semester for each course that a student takes. Can the object have multiple instances? If so, we must find a way to store data on each instance.
The cleanest way to do this is to represent the object as a separate entity. In our sales example, we must ask whether customers are allowed to have more than one email address; if they are, we should model the email address as a separate entity. Is the object often nonexistent or unknown? If so, it is effectively an attribute of only some of the entities, and it would be better to model it as a separate entity rather than as an attribute that is often empty.
Consider a simple example: The ER diagram representation of student grades as a separate entity Entity or Relationship? An easy way to decide whether an object should be an entity or a relationship is to map nouns in the requirements to entities, and to map the verbs to relations. All else being equal, try to keep the design simple, and avoid introducing trivial entities where possible; i.
Intermediate Entities It is often possible to conceptually simplify many-to-many relationships by replacing the many-to-many relationship with a new intermediate entity sometimes called an associate entity and connecting the original entities through a many-to-one and a one-to-many relationship. A passenger participates in an M: Any given flight can have many passengers with a booking.