tailieunhanh - Beginning Database Design- P17

Beginning Database Design- P17:This book focuses on the relational database model from a beginning perspective. The title is, therefore, Beginning Database Design. A database is a repository for data. In other words, you can store lots of information in a database. A relational database is a special type of database using structures called tables. Tables are linked together using what are called relationships. You can build tables with relationships between those tables, not only to organize your data, but also to allow later retrieval of information from the database | Creating and Refining Tables During the Design Phase a Western Union a Cash a Visa a MasterCard a American Express So the PAYMENT_METHODS field for a specific listing could be something like this Cashier s Check Western Union Visa MasterCard This string is a comma-delimited list. A comma-delimited list is by definition a multi-valued set. A multi-valued set is thus a set or a single item containing more than one possible value. 4NF demands that comma delimited strings should be split up. In the case of an online auction house it is likely that the PAYMENT_METHODS field would only be used for online display. Then again the list could be split in applications. For example the string value Visa determines that a specific type of credit card is acceptable perhaps processing payment through an online credit card payment service for Visa credit cards. 4NF would change the OLTP database model in Figure 10-18 to that shown in Figure 10-20. Seller_Payment_Methods seller_id FK Seller Category_Tertiary payment method Listing listi ng seller_id seller popularity_rating join_date address return_policy international tertiary_id tertiary seller_id FK tertiary_id FK secondary_id FK buyer_id FK description image start_date listing_days currency starting_price reserve_price buy_now_price number_of_bids winning_price Bid Seller_History seller_history_id buyer_id FK seller_id FK comment_date comments Buyer Buyer_History buyer_id 1 1 buyer_histor y_id buyer popularity_rating join_date address 1 Í seller_id FK buyer_id FK comment_date comments 4 bidder_id FK listing FK bid_price bid_date Figure 10-20 Applying 4NF to the OLTP database model. 293 Chapter 10 The sensibility of the application of 4NF as shown in Figure 10-20 depends on applications. Once again increasing the number of tables in a database model leads to more tables in query joins. The more tables there are in query joins the more performance is adversely affected. Using the 4NF application shown in Figure 10-20 a seller .