NET Support for Relational Databases in Visual Basic .NET

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122 NET Support for Relational Databases
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programming languages As we shall see, the NET framework takes this approach, while Visual Studio NET incorporates functional GUI tools as well
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12121 The Structure of a SQL statement
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A SQL statement is made up of three parts: 1 a verb, such as CREATE, MODIFY, SELECT, INSERT or UPDATE; 2 a predicate part that speci es a eld set to the query (eg *, TitlesAu_ID); 3 a prepositional clause that indicates the tables involved (eg Authors, Publishers) Typical queries are
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SELECT * FROM Authors; INSERT INTO Customers SELECT [New Customers]* FROM [New Customers];
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The NET framework does not specify or preclude any speci c form of SQL, since it is an express requirement that any database format can be used with it The various database drivers available to a Visual Basic program de ne how the program communicates with a database, and any speci c SQL that a programmer needs to embed in a piece of Visual Basic code must conform to that which the database expects For many, probably most, NET database programs, this will be TransactSQL, the implementation used by Microsoft s SQL server and MSDE
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1213 Relational Databases and Object-Orientation
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From our perspective as object-oriented programmers, a relational database is made up of tables, each of which can contain the member data for all of the objects of a speci c class Compared to a true object-oriented database, this is an inadequate system since each class s methods have been separated from the object data in a very non-object-oriented way However, we can make use of a relational database as a persistence mechanism Provided we use the information in tables as simply the data from which objects can be reconstituted, the relational database model works well in supporting object-oriented systems, particularly those in which objects are not required to be available on-line at all times
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NET Support for Relational Databases
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Microsoft provides in the NET framework a solid object hierarchy for the management of relational databases The range of classes for database work is found in the SystemData namespace For classes which must deal directly with a database
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12 n Databases in Visual Basic NET
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server (a program that manipulates a database on behalf of other programs), there are currently two categories; those that are optimized for use with SQL Server and MSDE, and those that can use Open DataBase Connectivity (ODBC) via Object Linking and Embedding (OLE) to provide access to a wide range of other database types including Microsoft Access, dBase III and IV, Oracle, DB2 and Paradox These are differentiated by a pre x, either Sql or OleDb in the class name Classes in the SystemData namespace without either of these pre xes can be used for working with data from either version A few classes are used to establish access to databases to support database working sessions (SqlConnection, OleDbConnection) while others provide ways of sending commands to a database server and retrieving the results (SqlCommand, OleDbCommand) Also important are the classes that encapsulate information retrieved from a database (SqlDataReader, OleDbDataReader) and those that allow that information to be updated (the SqlDataAdapter, OleDbDataAdapter and Dataset classes) These are backed up by a range of classes that provide detailed access to tables (DataTable), rows (DataRow), columns (DataColumn) and relations (DataRelation) The on-line help provided with Visual Studio NET is a comprehensive guide to these classes and their use
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1221 Three-Tier Application Support in NET
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Recall that the components of an application can be usefully organized into three separate tiers or layers This organization provides a good solution to the compromises often encountered in complex application design when considering the sometimes con icting goals of organizational complexity, application exibility, component re-use and maintenance We can build application-speci c and user-interface speci c code on top of the standard database classes to make it easy to manage data in programs The result is a three-tier application, the tiers being:
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The data-access layer, that deals with forming a connection with and getting information to and from the actual database Typically, the Sql/ OleDbConnection classes, the Sql/OleDbDataReader classes and the Sql/OleDbDataAdapter classes perform this function The business-object or business-logic layer that forms a set of business-objects from the information in the database so that certain business rules can be imposed on the way the information is handled An object model tailored to the system requirements is the most likely form of this layer The user-interface, or presentation layer, that deals with presenting the business objects and allowing the user to interact with them Forms and controls are used to implement this layer
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This three-tier approach gives us a very exible way of presenting a database to a user while imposing business rules on the way that the database is used The bene ts of this approach are the same bene ts that object-orientation can provide to any type of application, but have additional purchase in database style applications
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