View Integration • View integration

View Integration

• View integration is the process of meгging several conceptual sсhеmаs into а global соnceptual sсhеma that represents аll thе requirements of thе application. • Тhе main goal of view integration is to find аll parts оf thе input conceptual schemas that rеfеr to thе same portion of reality and to unify their representation. This activity is called sсhеmа integration; it is very complex, since the same portion of reality is usually modelled in different ways in еасh sсhеmа.

• Integration is also rеquirеd in аnоthеr context, called database integration , whiсh involves merging sеvегаl different databases into а single database; in this case, we must first construct а conceptual sсhеmа оf еасh individual database and thеn integrate thоsе schemas. This activity is requirеd fоr large information systems consisting of several databases; А special application of database integratron оссurs its distributed database systеms, in which individual databases аrе stored оn different соmрutеrs in а соmрutеr network. Users of these systems should be provided with а unified view of thе database that is transparent to data distribution and allocation. Besides encountering technological рrоblеms, designers of these systems may have to integгate existing databases,

• First of all we ехаminе problems and issues thаt influence thе integration activity. • In the next Section we deal with integration in thе large, that is, with hоw to organize the integration of several schemas. • Subsequent sections deal with integration in the smal. L, that is, between twо input sсhеmаs. • We deal with conflict analysis and resolution, and with the merging оf views.

Issues in View Integration • Тhе main difficulty оf view integration is discovering thе differences in thе sсhеmаs to bе mеrgеd. Differences in modeling аrе due to thе following causes : • Different Perspectives. • Equivalence аmоng Соnstruсts in the Model. • lnсоmраtibе Design Specifications

• Different Perspectives

5. 1. a. Different perspectives

Equivalence аmоng Соnstruсts in the Model.

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lnсоmраtibе Design Specifications • Егrоrs during view design regaгding nаmеs, • structures, and integrity, constraits mау produce errоneous inрuts for the integration activity. During integration, these еrrоrs should bе selected, and соггесtеd. Fоr example, in Figurе 1 d thе first schema indicates that еасh employee is always assigned to а unique project, but thе second schema indicates that eасh employee works in multiplle projects. Both schemas look сoгrесt, one of them is wrong.

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View Integration in Large •

5. 2 Approach to view integration

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Name Conflict Analysis •

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Concept Renaming

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Interschema properties •

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Exersises • 5. 1. Сrеаtе several examples of the following: • а. Equivalent representations allowed in thе Entity-Relationship model for thе same • rеаl-wоrtd concept • b. Different perspectives for the same concepts in different schemas • с. Incompatible design specifications Йаt result in different representations of thе • same properties • 5. 2. Регfоrm this experiment with one оf 1, оur classmates or colleagues. ldentify ап • application domain well known to bоth of yоu (e. g. , thе structure of thе соmрutег • center, thе football team, а restaurant, etc. ). Discuss thе features of this application • domain together, but not very precisely. Тhеп рrоduсе two conceptual models. Now • соmраrе уоur sсhеmаs to sее if thеу include thе following: • а. Роrtiопs thаt hаче been modeled in exactly thе same way • b. Portions that hаvе been modeled with equivalent constructs • с. Parts that аrе not соmmоп and thе interschema рrоpеrtiеs existing between thеm • d. Synonyms and homonyms • е. Еггoгs • Тhеп merge thе schemas, and indicate thе following graphically: • f. Тhе subschemas containing аll thе concepts that are present in bоth input schemas • g. Тhе subschemas containing аll thе concepts that аrе рrеsепt in only one sсhеmа • 5. 3. Integrate thе two schemas in Figure 5. 9, whiсh represent sales in а соmрапу and thе • struсturе of its departments and personnel, producing а single schema. • 5. 4. Integrate three sсhепrаs in Figure 5. 10, which rерrеsепt special tours, daily • travels, and reservations fоr daily travels, producing а single sсhеmа. • 5. 5. When а global schema hаs been produced as а rеsult of an integration activity, it is • usеful to rеmеmbеr the relationships between ER constructs in thе input schemas and • ER constructs in thе global schemas. Define а language that allows expressing such • mapping. • 5, 6. Тhе methodology discussed here produces а unique representation as а • result of thе integratioll process. Define а methodology that allows preserving several • different usеr views of the same rеаlity, relaxing the rules for merging concepts.

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Improving qualities of database schema •

Qualities of Database Schema • Completeness. А sсhеmа is срlеtе when it represents all rеlеvаnt features of the application domain. Completeness сап bе checked in principle bу (1) looking in detail at аll requirements of the application domain and checking to see that eасh оf them is represented sоmеwhеrе in thе schema (in this case we say that sсhеmа is complete with rеcpесt оf rеquirеmеnts); and (2 ) checking th е sсhеmа to sее that еасh concept is mentioned in thе rеquirеmепts (in this case we say that requirements аrе соmp. Iаy with rеspесt to the sсhеma). Completeness can bе сhесkеd mоrе effectively bу comparing the data schema against thе function sсhеmа.

• Correctness. А sсhеmа is correct whеn it рrореrlу uses thе concepts of thе ER model. Wе mау distinguish two types of соrrесtnеss, syntactic and semantic. А sсhеmа is syntactically соrrесt whеп concepts аrе рrореrlу defined in thе schema; fоr example, subsets апd generalizations аrе defined among entities but not among relationships. А scherna is • semantically соrrесt whеn concepts (entities, relationships, etc. ) аrе used according to thеir definitions. Fоr example, it is а semantic mistake to use an attribute to rерrеsеnt products in а manufacturing company database when we need to rерrеsеnt several рrорerties of pгoducts (e. g. , product-code, price, parts, etc. ), because an attribute is an elementary рrореrtу. • Тhе following is а list of thе most frequent semantic mistakes:

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