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