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The operations presented in Sec 7-4 are combined to query the database content A wide range of implementation approaches is possible for query processing For actual data retrieval issues of ease of use and generality become important These topics occupy most of Chap 9 Projection and selection expressions can be combined and excecuted similar to a join when formulating a query A query with selections and projection can be expressed in the form of a relation where the relation-schema is based on the list K and the tuples represent the selection list L An arti cial domain entry is now used to represent the notion any to indicate that a data element value or set of values is to be obtained
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Queries as Relations Example 7-12
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Selection is performed now using joins and projections applied to the query and data relations A query for name, birthdate on Employee who can weld becomes
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For general retrieval from a database selection expressions can include attributes from multiple relations When the programming language used does not handle general sets of records, sequences become important The information obtained from the databases has to be handled one record at a time, using the operations described in Sec 7-4-6 It may be possible to specify that we wish to loop through all the tuples in sequence Within the context of a current position within the sequence we can also reference the PRIOR, CURRENT, and NEXT tuple The concept of currency allows the speci cation of tuple-by-tuple algebraic procedures which would not be applicable to sets in general Some algebraic operations for tuple-by-tuple functions can also be applied to unsequenced relations, but the required generalization is not always obvious In many situations encountered in practice, the conceptual view model is based on natural sequences, and users tend to formulate queries oriented toward these sequential models
Programmed Queries An example of sequential processing is the computation of group subtotals within a relation which has as the ruling part a group attribute and a detail attribute, for instance, s id and p id in Supply of Fig 7-16 To determine the quantity of all parts supplied by one supplier we naturally add the quantities in sequence until the value of s id changes Then a new subtotal can be computed for the next supplier group
To obtain such subtotals in unsequenced relations is a function of the Group by clauses shown in Sec 9-2 A query-processing program can use the structural or a similar model to help the user in the formulation of queries by translating from the user s conceptual view model to the implemented database model
Sec 7-5 7-5
The Design of a Database Model
THE DESIGN OF A DATABASE MODEL
When a comprehensive set of view models has been established, the construction of a model for the entire database can be established Relations from separate view models may be combined based on the attributes which they have in common If view models have no attributes in common, there is no bene t in joining these data into a single database model Figure 7-25 sketches the concept
Even when there are common attributes there may be no connections A lack of connections indicates that the views or groups of views can be maintained independently of each other We will call a database created from views which do not connect to other databases an independent database An independent database is best maintained in a distributed manner, even if computer equipment is shared As we discussed in Sec 2-4-4, there are bene ts to distribution, and if databases can be left smaller and managed autonomously, total costs will probably be lower Forcing independent databases into one integrated database is sometimes done to permit retrieval queries to access the data from multiple independent databases Only a few database management systems today permit queries to be processed which access more than one database The cost of combining independent databases is increased database system overhead cost in order to provide the required view model independence and protection for update transactions Management costs, incurred to bring communality about in areas where there is little natural incentive to cooperate, may also be high Not even all connected view models should be integrated The linkage between some sets of views may be relatively weak and will not warrant the integration of a view model into the database A weak linkage may be due to a shared, but unchanging attribute In those cases we will also design independent databases, with a procedure to keep the shared attribute synchronized
If, for instance, employees are identi ed with a department, and production of goods with a department, then the list of departments may provide such a relatively constant link Only if employees are to be related to production will there be a su ciently strong coupling between the two areas to justify the combination of the view models
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