05 Transformation
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The document discusses rules for transforming entity-relationship models (ERMs) into relational schemas and tables. It outlines different relationship types between entity types A, B, and C and how they would be modeled as tables or attributes. Generalization/specialization relationships and triple relationships are also addressed. Transformation is not always unambiguous and different relationship types may be modeled in different ways as tables or attributes in the relational schema.
05 Transformation
- 1. Transformation of ERMs into Relational Schemas
- 2. Initial SituationInitial Situation: 3 Entity Types A, B und C
- 3. Transformation Rules ERM TablesTransformation RulesDescribethetransformationfrom ERM modelsintorelations (tables)Are not alwaysunambiguousSimple Example: (x,n)-(x,n)-Relation (x=0 v x=1)
- 4. (1,1)-(x,n)-Relation (x=0 v x=1)
- 5. (0,1)-(x,n)-Relation (x=0 v x=1)
- 6. (0,1)-(x,n)-Relation (x=0 v x=1)
- 9. (1,1)-(1,1)-RelationNote: Itisnecessarytohave a triggerfornewentriesthatenforces an entry in thesecondrelation, as well.
- 10. (1,1)-(1,1)-RelationAvoid (1,1)-(1,1) relationsBetter: Modelling as attributes
- 12. Remark: (1,x)-Relations (x={1,..,n})Example: (1,n)-(0,n)-Relations
- 13. Remark: (1,x)-Relation (x={1,..,n})Example: (1,n)-(0,n)-Relation
- 14. Remark: (1,x)-Relation (x={1,..,n})Example: (1,n)-(0,n)-Relation
- 15. Generalization/SpecializationHow will Generalization/Specialization be modelled in Relations, meaning in Tables?
- 20. Triple Relationship TypeEntries are arbitrary, as long as they are unique entries in A, B, and C.
- 21. Reinterpreted Relationship Type (1/2)New entries not arbitrary. Theyaredependenton alreadyexistingentries in AB!
- 22. Reinterpreted Relationship Type (2/2)Entries not arbitrary, because #C is Foreign Key
- 23. Transformation of ERMs into Relational Schemas