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PATTERNS OF DATA MODELING- P20

PATTERNS OF DATA MODELING- P20: Models provide the means for building quality software in a predictable manner. Models let developers think deeply about software and cope with large size and complexity. Developers can think abstractly before becoming enmeshed in the details of writing code. Although models are beneficial, they can be difficult to construct. That is where patterns come in. Patterns provide building blocks that help developers construct models faster and better. | 76 Chapter 5 Item Description Template SELECT ID.itemDescriptionID ID.name FROM Item AS I INNER JOIN ItemDescription AS ID ON I.itemDescriptionID ID.itemDescriptionID WHERE I.itemID anItemID Figure 5.3 Item description SQL query. Find the description given an item. SELECT I.itemID I.name FROM Item AS I INNER JOIN ItemDescription AS ID ON I.itemDescriptionID ID.itemDescriptionID WHERE ID.itemDescriptionID anItemDescriptionID ORDER BY I.name Figure 5.4 Item description SQL query. Find the items for a description. ItemDescription table item DescriptionID name 11 1986 Ford Escort 12 1989 Mazda 323 Item table item ID name item DescriptionID 1 VIN 1032547698 11 2 VIN5724916835 11 3 VIN 3761952953 11 4 VIN 1123581321 12 Figure 5.5 Item description Populated tables. 5.1.5 Examples The item description template occurs frequently. Of all the templates it is the one you will find most often. Figure 5.6 shows the nucleus of a model for motor vehicles. VehicleModel corresponds to ItemDescription and PhysicalVehicle corresponds to Item. Physical vehicles are items with individual identification numbers. In contrast vehicle models refer to the year and make such as 1986 Ford Escort and 1989 Mazda 626. Customer repair records refer to physical vehicles while design documents describe vehicle models. The library loan model in Figure 5.7 further demonstrates the template. A Libraryitem is something that a library offers for borrowing. A LibraryitemCopy is a physical copy of a LibraryItem. For example a library may list The Grapes of Wrath in its card catalog a LibraryItem and have five copies of the book available LibraryItemCopies . 5.1 Item Description Template 77 Figure 5.6 Item description Vehicle model. Figure 5.7 Item description Library loan model. Each category of Libraryitem LibraryltemType has a standard checkout period and number of renewals. For example children s books may be checked out for a month while adult books may be checked out for only two weeks. A Libraryitem