What is an Entity-Relationship Model (ER Model)? - Definition from Techopedia
Entities, defined as tables that hold specific information (data); Relationships, defined . In the entity relationship diagram, shown in Figure , each attribute is. Attributes are not shown on the Entity-Relationship Model but are recorded in the underlying data dictionary which contains the definitions of. For our sales database example, the product and customer entity sets would be . Like entities, relationships can have attributes: we can define a sale to be a.
The solution is to either adjust the model or the SQL.
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This issue occurs mostly in databases for decision support systems, and software that queries such systems sometimes includes specific methods for handling this issue. The second issue is a 'chasm trap'.
A chasm trap occurs when a model suggests the existence of a relationship between entity types, but the pathway does not exist between certain entity occurrences. For example, a Building has one-or-more Rooms, that hold zero-or-more Computers.
One would expect to be able to query the model to see all the Computers in the Building. However, Computers not currently assigned to a Room because they are under repair or somewhere else are not shown on the list. Another relation between Building and Computers is needed to capture all the computers in the building.
This last modelling issue is the result of a failure to capture all the relationships that exist in the real world in the model.
See Entity-Relationship Modelling 2 for details. 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. A popular approach to conceptual design uses the Entity Relationship ER model, which helps transform the requirements into a formal description of the entities and relationships that appear in the database. In the ER diagram, an entity set is represented by a rectangle containing the entity name.
An entity set is represented by a named rectangle We typically use the database to store certain characteristics, or attributes, of the entities.
In a sales database, we could store the name, email address, postal address, and telephone number for each customer.PGC Lectures: Data Modeling, Entity, Attribute, Relationship
Attributes describe the entity they belong to. 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.
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.
What is Entity Relationship Diagram? Webopedia Definition
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.
Entity–relationship model - Wikipedia
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. 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.