Tham khảo tài liệu 'an introduction to database systems 8ed - c j date - solutions manual episode 1 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | R assuming the INSERTS and DELETES all succeed of course. Again if we decide to treat join views in some special way then consistency dictates that we treat EACH AND EVERY relational operator in its own special way special rules for union special rules for divide and so on. Everything becomes a special case in fact consistency dictates inconsistency . This surely can t be a good idea. Of course it s essentially what today s DBMSs all do insofar as they address the problem at all. The net of all this is that one simple rule that applies in all cases is surely the right way to go. Especially since in the example of S JOIN SP we can achieve the desired DELETE behavior by applying the DELETE direct to relvar SP instead of to the join view Of course nothing in the foregoing argument precludes the possibility of placing logic in application code sitting on top of the DBMS that a allows the join to be displayed as a single table on the screen b allows the end user to remove a row from that table somehow and c implements that removal by doing a DELETE on relvar SP only under the covers. But we must avoid any suggestion that what the end user would be doing in such a scenario is a relational DELETE. It s a different operation and the user would need to understand that fact in general it has different semantics and it should be given a different name. The relational model consists of five components 1. An open-ended collection of scalar types including in particular the type boolean or truth value Comment The scalar types can be system- or user-defined in general thus a means must be available for users to define their own types this requirement is implied partly by that open-ended . A means must therefore also be available for users to define their own operators since types without operators are useless. The only built-in . system-defined type we insist on is type BOOLEAN but a real system will surely support integers strings etc. as well. 2. A relation type .
