ERM
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22 terms
Terms | Definitions |
|---|---|
 data modelling | focuses on static aspects |
| entity | thing |
| entity type | set of similar things |
| relationship | conects two or more entities |
| relationship type | set of similar relations |
| cardinality | maximun number of entities that an entity can be conected to |
| attribute | assigns a value from a domain to every entity of an entity set |
| domain | - range of all allowed values - basic data types |
| types of attributes | - key attribute (unique) - multivalue attribute - composite attribute - derived attribute |
| comments | - explain something |
| constraints | - prevent interpretatitions that would hamper the system's integrity |
| specialisation | - defining a subst form a superset (of an entity type) |
| generalisation | - defining a superset based on several sets |
| supertype | generalised entity type |
| subtype | specialised entity type |
| completeness constraint | - describes, whether an entity has to belong to a subset - total (yes) - partial (no) |
| disjoint constraint | - describes, wheter an entity can belong to more than just one subset - overlapping (yes) - disjoint (no) |
| assesment of generalisation/specialisation | - supports different views - reduction of complexity - needs to be analyzed concerning potential changes over time |
| circles | may lead to inconsitencies |
| ERM advantages | - enables the abstraction from concrete database technologies - promotes an intuitive visualisation of data structures - high diffusion |
| ERM limitations | - limited set of data types to specify attributes - no reading direction for relationship types - traditional notation hampers readabliity - no common interpretation of generalisation/specialisation |
| weak entity type | - dependent - cannot exist without other entitytyps |
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