Datenbankmodelle Definition 2 (Datenbankmodell, Datenbankschema Einführung in das Entity-Relationship-Modell Die Beschreibung der Informationsstruktur eines Anwendungsbereichs ER-Konzepte und ihre Semantik Attribute von Entity-Typen .. Mehrwertige Abhängigkeiten Definition 13 (mehrwertig abhängig). Definition, The Entity-Relationship (ER) model, a high-level data model that is ER diagram notation for an attribute domain (StudentGrade) of an entity type. For example an employee entity may have the attributes Name, SSN,. Address, Sex . ER model has three main concepts: ▻ Entities (and A relationship relates two or more distinct entities with a specific meaning. .. Mehrwertige Attribute.
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.
Entity–relationship model - Wikipedia
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".13.10 Entity Relationship Modellierung III: schwache Entitätstypen, N:M, Generalisierung, Teil-von
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. 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.
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. Unfortunately, Itnatios Trekas had to drop the course and retake it another semester. Consequently, his name is associated with two team numbers.
We arrange the diagram so that the relationship reads from left to right, "a student is a member of a team. Definition A relationship type is a set of associations among entity types.
A relationship or relationship instance is an ordered pair consisting of particular related entities. The degree of a relationship type is the number of entity types that participate. Thus, the LabSecMemberOf relationship type of Figure 9 has degree 2, which we call a binary relationship type. To clarify the role that an entity plays in each relationship instance, we can label a connecting edge with a role name that indicates the purpose of an entity in a relationship.
For example, we can have two binary relationship types associating the student and team types, TeamMemberOf and LeaderOf.
In the former case, a student entity is a member of a team entity; in the later case, a student can be a leader of a team. We illustrate the situation in Figure Definition The degree of a relationship type is the number of entity types that participate.
If two entity types participate, the relationship type is binary. A role name indicates the purpose of an entity in a relationship. ER diagram notation with roles member, leader, and lab team As Figure 11 illustrates, a relationship type can also have attributes. The relationship type order connects entities chemical and supplier.
The relationship is many-to-many because each chemical can be from several suppliers and each supplier has a number of chemicals. An order has a purchase date, amount, and total cost as well as the chemical and supplier information. Thus, order has attributes PurchaseDate, amount, and TotalCost that we cannot appropriately associate with chemical or supplier.