英语翻译Figure1-3showsthehierarchicalstructureofthehierarchicalmodelfortheNorthwindTradersdatabase.YouwillrecognizetheCustomer,Employee,Product,Order,andOrderDetailrecordtypesastheywereintroducedpreviously.Comparingthe

Question

英语翻译
Figure 1-3 shows the hierarchical structure of the hierarchical model for the Northwind Traders database.You will recognize the Customer,Employee,Product,Order,and Order Detail record types as they were introduced previously.Comparing the hierarchical structure with the flat file system shown in Figure 1-2,note that the Employee and Product records are shown in the hierarchical structure with dotted lines because they cannot be connected to the other records via pointers.These illustrate the most severe limitation of the hierarchical model that was the main reason for its early demise:No record can have more than one parent.Therefore,we cannot connect the Employee records with the Order records because the Order records already have the Customer record as their parent.Similarly,the Product records cannot be related to the Order Detail records because the Order Detail records already have the Order record as their parent.Database technicians would have to work around this shortcoming either by relating the "extra" parent records in application programs,much as was done with flat file systems,or by repeating all the records under each parent,which of course was very wasteful of then-precious disk space-not to mention the challenges of keeping redundant data synchronized.Neither of these was really an acceptable solution,so IBM modified IMS to allow for multiple parents per record.The resultant database model was dubbed the extended hierarchical model,which closely resembled the network database model in function,as discussed in the next section

Answer 1

图1-3显示的层次结构层次模型为Northwind交易员的数据库.你会认出顾客、员工、产品、秩序、订单细节记录类型,因为他们介绍之前.比较平坦的层次结构的文件系统,如图1-2,需要注意的是,雇员和产品的记录显示在层次结构,因为他们无法用点线连接到其他记录通过指针.这说明最严重缺陷分层模型的主要原因,是它的早期死亡:没有记录可以有多个家长.因此,我们不能把员工记录的排列顺序纪录,因为订单已记录的客户作为记录他们的父母.同样,该产品记录不能被相关的详细记录,因为订单订购详细记录已经命令作为记录它们的父亲.数据库技术人员将不得不在这个缺点,要么以“额外的“家长记录在应用程序时,多与平面文件系统,或通过重复所有记录下每一个家长,这当然是很浪费then-precious磁盘space-not提到的数据冗余的挑战的同步.这两种真的是一个可接受的解决方案,以允许国际商用机器公司(IBM)修改相关的多重的父母/记录合成数据库模式被称为扩展层次模型,该模型的网络数据库非常类似的功能,在第二节.