Entity–relationship model - Wikipedia
The entity relationship (ER) data model has existed for over 35 years. It is well An entity is considered strong if it can exist apart from all of its related entities. For example, in the COMPANY database, a Spouse entity is existence - dependent on the Employee entity. These entities have the following characteristics. Subgroups have some different attributes, not shared by entire entity-set. • E-R model provides specialization to represent such entity-sets. • Example: bank. The world that we live in can be abstracted out as a simple model which can be used to solve many existing problems. The primary concept of.
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. It is important to identify relationships correctly, since that is what explains the logic behind the entities in a given domain. In basic terms, the association between entities can be referred to as a relationship. Introduction to Entity-Relationship Model Diagram An Entity Relationship Model, which is also known as a ER Model, explains the relationships between objects entitieswithin a given domain of interest.
An entity relationship model consists of three major components. Entity Relationship Cardinality Every ER Model has entities and relationships between these entities, that explains how the entities are connected to each other.
An example ER Diagram is shown below. An ER Diagram, is a basic flowchart that identifies how entities in a given domain, interact with each other as a flow of events. The cardinality of ER diagrams is important, as it shows the number of entities which are connected to another selected entity. It makes the life easy for the programmer as well as the reader, to understand the type of relationship each entity has, over others.
Let's now have a look at how cardinality works in ER Diagrams. An attribute may be formed from smaller parts; for example, a postal address is composed of a street number, city, ZIP code, and country.
Some attributes can have multiple values for a given entity. For example, a customer could provide several telephone numbers, so the telephone number attribute is multivalued. Attributes help distinguish one entity from other entities of the same type. We could use the name attribute to distinguish between customers, but this could be an inadequate solution because several customers could have identical names.
To be able to tell them apart, we need an attribute or a minimal combination of attributes guaranteed to be unique to each individual customer. The identifying attribute or attributes form a key.
In our example, we can assume that no two customers have the same email address, so the email address can be the key. However, we need to think carefully about the implications of our choices. For example, if we decide to identify customers by their email address, it would be hard to allow a customer to have multiple email addresses.
The Entity Relationship Model - Learning MySQL [Book]
Any applications we build to use this database might treat each email address as a separate person, and it might be hard to adapt everything to allow people to have multiple email addresses.
Clearly, there may be several possible keys that could be used to identify an entity; we choose one of the alternative, or candidate, keys to be our main, or primary, key. You usually make this choice based on how confident you are that the attribute will be non-empty and unique for each individual entity, and on how small the key is shorter keys are faster to maintain and use. Attributes comprising the primary key are shown underlined. The parts of any composite attributes are drawn connected to the oval of the composite attribute, and multivalued attributes are shown as double-lined ovals.
Similarly, a product price could be a positive rational number. Attributes can be empty; for example, some customers may not provide their telephone numbers.
You should think carefully when classifying an attribute as multivalued: The sales database requirements may specify that a product has a name and a price. To distinguish between products, we can assign a unique product ID number to each item we stock; this would be the primary key. 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.
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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.