What is Entity Relationship Diagram? Webopedia Definition
The Entity Relationship Model At a basic level, databases store information it in Entity Relationship Modeling Examples” for three sample databases. We typically use the database to store certain characteristics, or attributes, of the entities. A Person can have a relationship with a Bank of either being a Debtor or Creditor . The relationships in an E-R model are explicitly defined, while they are. This definition explains the meaning of entity relationship diagram, also known as ERD, After a relational database is rolled out, an ERD can still serve as a referral point, ERDs are generally depicted in one or more of the following models.
February In using a modeled database, users can encounter two well known issues where the returned results mean something other than the results assumed by the query author. The first is the 'fan trap'.
Entity–relationship model - Wikipedia
It occurs with a master table that links to multiple tables in a one-to-many relationship. The issue derives its name from the way the model looks when it's drawn in an entity—relationship diagram: This type of model looks similar to a star schemaa type of model used in data warehouses.
When trying to calculate sums over aggregates using standard SQL over the master table, unexpected and incorrect results. The solution is to either adjust the model or the SQL. This issue occurs mostly in databases for decision support systems, and software that queries such systems sometimes includes specific methods for handling this issue.
ER Model - Basic Concepts
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".
So, it is a one to one relationship. When one instance of an entity is associated with many instances of another type of entity, it is called one to many 1: In a faculty, there can be multiple students, but a single student is in one faculty.
Therefore, it is a one to many 1: When many instances of an entity are associated with many instances of another type of entity, it is called many to many m: A student can participate in multiple courses, and a single course can have multiple students.
So, it is a many to many relationship m: Furthermore, when an entity is related to itself, it is called a recursive relationship. When there are three entities connected, that relationship is known as a ternary relationship. In the above ER diagram, the Lecture conducts Course. A single Lecturer can conduct multiple Courses, but one Course is conducted by one Lecturer.
So, it is a one to many 1: A Student can follow multiple Courses, and a single Course can have multiple Students. Therefore, it is a many to many m: The Course has an Exam. The Exam is a weak entity, and it depends on the Course. So, that relationship is a weak relationship.
It is denoted by a double diamond symbol. A relationship is an association among entities. A double rectangle represents a weak entity. A rhombus or diamond represents a relationship in an ER diagram. A double rhombus denotes a weak relationship. Usage The entities help to represent real-world objects. They are the tables.