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Geo-Databases: lecture 7 Database design

Geo-Databases: lecture 7 Database design. Prof. Dr. Thomas H. Kolbe Institute for Geodesy and Geoinformation Science Technische Universität Berlin.

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Geo-Databases: lecture 7 Database design

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  1. Geo-Databases: lecture 7Database design Prof. Dr. Thomas H. Kolbe Institute for Geodesy and Geoinformation Science Technische Universität Berlin Credits: This material is mostly an english translation of the course module no. 8 (‘Geo-Datenbanksysteme‘) of the open e-content platform www.geoinformation.net.

  2. Database design

  3. Motivation (1) • Desired: A suitable relational schema in order to describe a given application (mini-world) • Role of the database design: Systematic development of database schemata, with regard to the specific information needs of the user. • Starting with SQL right away has some disadvantages: • Poor in structuring concepts • Application semantics therefore hidden in tables, foreign keys, etc • Effects of changes in the real world on the schema hardly traceable • Instead, we may perform the design process on a conceptual level: • Usage of an abstract modelling language (e.g. Entity-Relationship-diagrams, UML-models) • Projectable onto the language of the DBMS, e.g. an SQL schema

  4. Database design– a miniature example • Task: Examination database Problem: Projection of an object-model into a relational schema?

  5. Logical design / Implementation • A conceptual model can be converted into an implementable database model.

  6. Translation into the relational model There are some “cooking recipes“, that can help with the translation of a UML-diagram into a relational schema. • Generally, the translation process can be carried out in several steps: • Each class, that does not take part in the inheritance hierarchy is projected onto a table • Each association, besides inheritance, is projected onto a table • Refinement: Some tables can be combined • Separate treatment of inheritance hierarchies

  7. Translation of classes • Each class is projected onto a table. • Name of the table: name of the class • Attributes: • Attributes of the class in the UML-diagram • Note: if necessary, UML-data types have to be adapted to the SQL-data types! • Example:

  8. Translation of associations • Each association is projected onto a table. • Name of the table: name of the association or, if existent, of the related association class; otherwise meaningful, new name • Attributes: (if necessary attributes might have to be re-named!) • Key attributes of the involved classes • Attributes of the related association class (if existent) • Example: 1:n-relationship between class room and class lecture

  9. key of room for lecture Refinement: Combination of tables • Some of the tables, that emanate from associations, can be eliminated. Let A and B be the classes / tables of the association. • The options for the combination are determined by the type of association. • 1:1 The key of B can be added to table A or vice versa. The association table can be eliminated • 1:n The key of A can be added to the table B (“n-side“). The association table can be eliminated • n:m The association table should persist.

  10. Translation of inheritance hierarchies • When it comes to inheritance hierarchies only the concerned classes are translated but not the inheritance relationships. • There are several reasonable alternatives: • One table per class in the hierarchy • One table for each partial tree of the hierarchy that emanates from the root • A single table for the entire hierarchy • Criteria for the selection of an alternative: • Required space (tuple size and number of emerged tables) • Support of important manipulation operations and queries for the given application

  11. inheritance hierarchies, alternative I • Alternative I: One table per class in the hierarchy. For each table: • Name of the table: name of the class • Attributes: • Attributes of the corresponding class in the UML-diagram • Key attributes of the root in the hierarchy, that remain keys

  12. PersonsEmpAssStud Street wage inheritance hierarchies, alternative II (1) • Alternative II: A table for each partial tree of the hierarchy that emanates from the root. For each partial tree: • Name of the table: meaningful, clear name • Attributes: al attributes of the concerned classes, renamed if necessary • Tables: • Persons, • PersonsEmp, • PersonsEmpProf, • PersonsEmpAss, • PersonsEmpProfAss, • PersonsStud, • PersonsEmpStud, • PersonsEmpAssStud, • PersonsEmpProfStud, • PersonsEmpProfAssStud

  13. inheritance hierarchies, alternative II (2) • Alternative II illustrates the subject of disjunctive / non-disjunctive inheritance. • An inheritance hierarchy is called non-disjunctive, if an object of an upper class K may have the type of more than one lower class of K.In our Example: Person is employee and (phd-) student at the same time • Non-disjuntive inheritance can be expressed in all of the presented translation alternatives • Alternative II shows an analogy to object-oriented programming languages in terms of disjuntive inheritance: All attributes of an object are stored in a “coherent“ memory area. • If an inheritance hierarchy was translated according to alternative II, it requires the following amendments: • Eliminate all tables that represent nonsensical combination in terms of the given application!In our Example: PersonsEmpProfStud

  14. inheritance hierarchies, alternative III • Alternative III: A single table for the entire hierarchy. • Name of the table: for example class name of the root • Attributes: all class attributes that appear in the hierarchy, renamed if necessary. Not corresponding attributes are set to NULL!

  15. inheritance hierarchies, comparison (1) • Comparison of the alternatives on the basis of the previously mentioned size parameters and the example query: “show name and rank of all professors!“

  16. inheritance hierarchies, comparison (2) • Comparison of the alternatives on the basis of the dominating query types in the given application. Dominating attribute queries of the application + / - : number of tables to be included low / high

  17. Refinement of the design • The implementable database schema can be improved: • In terms of redundancy: • Enforcement of the so-called normalisation of a relational database schema (later!) • In terms of performance: • controlled redundancy by targeted denormalisation • by specifyfing further physical characteristics of the database, such as index structures for efficient access (see also “B-tree“ in LE 8 and “R-tree“ in LE 11)

  18. References Overview: • Hector Garcia-Molina, Jeffrey D. Ullman, Database Systems: The Complete Book, Prentice Hall, 2002 • Bernd Oestereich, Objektorientierte Softwareentwicklung: Analyse und Design mit der Unified Modeling Language, Oldenbourg Verlag, München, 1998 • Gottfried Vossen, Datenbankmodelle; Datenbanksprachen und Datenbankmanagement-Systeme, Oldenbourg Verlag, München, 1999 Original works: • P.P.-S. Chen, The entity-relationship model: Toward a unified view of data, in: ACM Transactions on Database Systems, Vol. 1, Seite 9-36, 1976 • J. Rumbaugh, I. Jacobson, G. Booch, The Unified Modeling Language for Object-Oriented Development - Documentation Set 0.9 Addendum, Rational Software Corporation, Santa Clara 1996

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