Building a Data Model in .NET framework

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Building a Data Model
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If desired, you can build a data model that illustrates your database design A data model is a visual representation of the database needed by the
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Data Design
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application (or unit of work) that depicts the tables with their associated elds It also shows the relationships between the tables The easiest way to build a data model is to use a data modeling tool Visual Studio provides such a tool, as detailed in 8 Figure 27 shows the resulting data model for the Purchase Tracker application (current unit of work)
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2 DESIGNING SOFTWARE
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Figure 27 The data model for the Purchase Tracker database de nes the tables and elds required by the Purchase Tracker application
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NOTE: The Customer table in the Purchase Tracker example contains an address with City, State, and Zip columns Because the city and state depend on the zip code, you may consider this table to violate third normal form However, it is left as is to simplify the coding examples later in this book
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Tuning for Performance
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With normalized tables often comes the need to perform joins A join is the combination of records based on a de ned matching eld, most often a key eld For example, the Customer and Purchase tables could be joined on the CustomerID to nd all the purchases made by a speci c
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customer As you may guess, performing joins requires some processing time Performing complex multijoins requires noticeable processing time So you may need to tune your database for performance You may nd that a particular frequently used join is too slow To improve the performance, you can consider adding indexes or denormalizing the table This may involve adding calculated or redundant elds in the record, breaking the normalization rules Document any speci c changes to the database for performance, especially if it breaks normalization rules, in the data design document
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Defining the Data Access Strategies
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After you know how the data is stored, you need to de ne how the application performs create, read, update, and delete (CRUD) operations on that data The most common technique for working with data in a database is to build stored procedures (called queries in Microsoft Access) You can build one stored procedure for each CRUD operation Or you can build one stored procedure for read and one for the create, update, and delete operations The data access layer of your application can use ADONET, the NET Framework data access library, to call the stored procedures and retrieve data or perform maintenance operations This is demonstrated in 8 There are situations in which a stored procedure cannot be used to access data in a database For example, if the data required by the application is too ad hoc, it may be dif cult to write a stored procedure that is suf ciently generalized In this case, you can write SQL statements directly in your application and use ADONET to execute them
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NOTE: Using SQL statements directly in your application is not recommended for performance and security reasons Use stored procedures (QueryDefs in Microsoft Access) instead
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For data not stored in a database, such as con guration les, the NET Framework provides classes to assist you in retrieving and updating data The code to retrieve and update user con guration settings is presented in 4
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Defining How to Provide Data to Other Layers
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To implement a three-tiered architecture, you can design your application to separate the user interface from the business logic from the data accessing Your data access class retrieves the data You are then faced with the question of how to transfer that data to the other layers You need to de ne how the data access class provides the data to the business logic layer and how the business logic layer provides the data to the user interface layer You have many options for getting data from one part of your application to another Here are several common options:
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Pass a DataSet A DataSet is an ADONET data structure that contains the results of one or more queries accessible as rows and columns For this option, the data access class retrieves data from the database directly into a DataSet and passes it to the business logic layer The business logic layer reviews/modi es the DataSet to impose necessary logic In some cases, this can be done with the DataSet s extended properties The DataSet is then passed to the user interface layer, which gets the elds from the DataSet and displays them The user interface code to populate a textbox would be something like this:
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