This page describes the three types of relationships when examining how entities relate to one another in a data model. A relationship type is illustrated in an ERD using a diamond symbol. Consider Below is a ternary relationship (a relationship type involving three entity types). An entity–relationship model (ER model for short) describes interrelated things of interest in a A basic ER model is composed of entity types (which classify the things of interest) and specifies relationships that .. model looks when it's drawn in an entity–relationship diagram: the linked tables 'fan out' from the master table.
The first is the 'fan trap'. 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.
N-ary relationship types
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.
This example can be implemented very easily.Attributes to relationships
In the diagram below, we represent our ternary relationship with an extra table, which can be modelled in Vertabelo very quickly. In other words, a group can have specific classess only at one classrom. Sometimes it is possible to replace a ternary or n-ary relationship by a collection of binary relationship connecting pairs of the original entities. However, in many cases it is hard to replace ternary relationship with two or more binary relationships because some information could be lost.
Another ternary relationship presents a different situation — Teacher recommends a book for a class: In the example with groups and classes, the primary key consisted only of two foreign keys.
This meant that there could be only one classroom for a specific group and class. In this situation the primary key consists of all three foreign keys.
Entity–relationship model - Wikipedia
It makes a significant difference, because in this case it is possible to recommend more than one book for a specific course by one teacher. It is possible that the teacher recommended the book for another class. We can tell who recommends the book and which book is used in which class. Unfortunately, we lack information on who exactly recommended that book for that class; it could have been done recommended by another teacher.