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create procedure ndEmpl(in mgr char(10)) Finds all employees who work directly or indirectly for mgr and adds them to the relation empl(name) The relation manager(empname, mgrname) speci es who directly works for whom begin create temporary table newemp (name char(10)); create temporary table temp (name char(10)); insert into newemp select empname from manager where mgrname = mgr repeat insert into empl select name from newemp; insert into temp (select managerempname from newemp, manager where newempempname = managermgrname; ) except ( select empname from empl ); delete from newemp; insert into newemp select * from temp; delete from temp; until not exists (select * from newemp) end repeat; end Figure 95 Finding all employees of a manager
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employees of mgr, and stores them in the temporary table temp Finally, it replaces the contents of newemp by the contents of temp The repeat loop terminates when it nds no new (indirect) employees We note that the use of the except clause in the procedure ensures that the procedure works even in the (abnormal) case where there is a cycle of management For example, if a works for b, b works for c, and c works for a, there is a cycle While cycles may be unrealistic in management control, cycles are possible in other applications For instance, suppose we have a relation ights(to, from) that says which
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cities can be reached from which other cities by a direct ight We can modify the ndEmpl procedure to nd all cities that are reachable by a sequence of one or more ights from a given city All we have to do is to replace manager by ight and replace attribute names correspondingly In this situation there can be cycles of reachability, but the procedure would work correctly since it would eliminate cities that have already been seen
97 Object-Oriented versus Object-Relational
We have now studied object-oriented databases built around persistent programming languages, as well as object-relational databases, which are object-oriented databases built on top of the relation model Database systems of both types are on the market, and a database designer needs to choose the kind of system that is appropriate to the needs of the application Persistent extensions to programming languages and object-relational systems target different markets The declarative nature and limited power (compared to a programming language) of the SQL language provides good protection of data from programming errors, and makes high-level optimizations, such as reducing I/O, relatively easy (We cover optimization of relational expressions in 13) Objectrelational systems aim at making data modeling and querying easier by using complex data types Typical applications include storage and querying of complex data, including multimedia data A declarative language such as SQL, however, imposes a signi cant performance penalty for certain kinds of applications that run primarily in main memory, and that perform a large number of accesses to the database Persistent programming languages target such applications that have high performance requirements They provide low-overhead access to persistent data, and eliminate the need for data translation if the data are to be manipulated by a programming language However, they are more susceptible to data corruption by programming errors, and usually do not have a powerful querying capability Typical applications include CAD databases We can summarize the strengths of the various kinds of database systems in this way: Relational systems: simple data types, powerful query languages, high protection Persistent-programming-language based OODBs: complex data types, integration with programming language, high performance Object-relational systems: complex data types, powerful query languages, high protection These descriptions hold in general, but keep in mind that some database systems blur the boundaries For example, some object-oriented database systems built around a persistent programming language are implemented on top of a relational database system Such systems may provide lower performance than object-oriented database systems built directly on a storage system, but provide some of the stronger protection guarantees of relational systems
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