database - relationship attributes in ER diagrams - Stack Overflow
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. By defining the entities, their attributes, and showing the relationships between Cardinality specifies how many instances of an entity relate to one instance of. An attribute is never linked to an entity via a relationship. A relationship links two entities. It does not link entities with attributes. Attributes describe an entity.
A particular value of an attribute, such as 93 for the grade, is a value of the attribute. Most of the data in a database consists of values of attributes. The set of all possible values of an attribute, such as integers from 0 to for a grade, is the attribute domain.
In an ER model, an attribute name appears in an oval that has a line to the corresponding entity box, such as in Figure 3.
Definition An attribute of an entity is a particular property that describes the entity. The set of all possible values of an attribute is the attribute domain.
Sometimes the value of an attribute is unknown or missing, and sometimes a value is not applicable. In such cases, the attribute can have the special value of null. For example, until the professor grades a laboratory assignment, the team grade is missing or null. Definition Null is the special attribute value that indicates an unknown or missing value. An attribute can be simple or composite.
A simple attribute, such as grade, is one component that is atomic. If we consider the name in two parts, last name and first name, then the name attribute is a composite. A composite attribute, such as "Emanuel Vagas", has multiple components, such as "Emanuel" and "Vagas"; and each component is atomic or composite.
We illustrate this composite nature in the ER model by branching off the component attributes, such as in Figure 4.
Definition A simple attribute is one component that is atomic. A composite attribute has multiple components, each of which is atomic or composite. ER diagram notation for composite attribute domain, name Another way to classify attributes is either as single-valued or multi-valued.
For an entity an attribute, such as StudentGrade, usually holds exactly one value, such as 93, and thus is a single-valued attribute. However, two lab assistants might assist in a laboratory section. Consequently, the LabAssistant attribute for the entity LabSection is multi-valued. A multi-valued attribute has more than one value for a particular entity.
We illustrate this situation with a double oval around the lab assistant type, LabAssistant see Figure 5.
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Definition For a particular entity, an entity attribute that holds exactly one value is a single-valued attribute. ER diagram notation for multi-valued attribute domain, LabAssistant A derived attribute can be obtained from other attributes or related entities.
For example, the radius of a sphere can be determined from the circumference. We request the derived attribute with a dotted oval and line, such as in Figure 6. ER diagram notation for derived attribute, radius An attribute or set of attributes that uniquely identifies a particular entity is a key. However, to determine the class we need a composite key that consists of several attributes, such as catalogue number, section, semester, and year. In the ER diagram of Figure 7we underline the composite key, class.
The figure shows another attribute DragExpWeek of LabSection that stores the week of the semester in which the drag experiment occurs. Definition An attribute or set of attributes that uniquely identifies a particular entity is a key. A composite key is a key that is a composite of several attributes. For example, the student entity type is related to the team entity type because each student is a member of a team. In this case, a relationship or relationship instance is an ordered pair of a specific student and the student's particular physics team, such as Emanuel Vagas, PhysFA04where PhysFA04 is Emanuel's team number.
Figure 8 illustrates three relationships.
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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.
entity-relationship diagram (model)
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.