DBMS Keys: Primary, Candidate, Super, Alternate and Foreign (Example)
DBMS ER Model Basic Concepts - Learn DBMS in simple and easy steps starting Super Key − A set of attributes (one or more) that collectively identifies an entity in Primary Key − A primary key is one of the candidate keys chosen by the. Where there is more than one set of attributes which could be chosen as the primary key for an entity, each of these groups of. Structure of Relational Databases; Convert a ER Design to a Relational Database Table EM has attributes corresponding to the primary key of E and an of the primary keys of the related entity sets becomes a super key of the relation.
In other situations there may be more than one candidate key for a relation, and no candidate key is obviously preferred.
A surrogate key may be used as the primary key to avoid giving one candidate key artificial primacy over the others. Since primary keys exist primarily as a convenience to the programmer, surrogate primary keys are often used, in many cases exclusively, in database application design. Due to the popularity of surrogate primary keys, many developers and in some cases even theoreticians have come to regard surrogate primary keys as an inalienable part of the relational data model.
This is largely due to a migration of principles from the object-oriented programming model to the relational model, creating the hybrid object-relational model. In the ORM like active record patternthese additional restrictions are placed on primary keys: Primary keys should be immutable, that is, never changed or re-used; they should be deleted along with the associated record.
Primary keys should be anonymous integer or numeric identifiers.
Entity–relationship model - Wikipedia
However, neither of these restrictions is part of the relational model or any SQL standard. Due diligence should be applied when deciding on the immutability of primary key values during database and application design.
Other candidate keys become alternate keys, each of which may have a unique index assigned to it in order to prevent duplicates a duplicate entry is not valid in a unique column. The first is the 'fan trap'.
Primary key - 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. 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.
Example: Relational 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.