Beginning Visual Basic .NET Database Programming phần 3 ppt

Chapter 5 4 A Brief Introduction to ADO.NET Before we can begin creating our application we need to learn about some basic components of ADO.NET and wizards that our application is to use. We will go into greater detail on ADO.NET in the next chapter so, for now, we'll learn just enough to get us through this simple database project. ADO.NET provides us with a way of gathering data and information and presenting it through a user interface. By using some components, we're able to connect to various data sources and can then build a user interface that accesses a database. We need four pieces to build our ADO.NET project: 1. A data source – where the actual data is stored, our database. 2. A Connection object – for connecting us to our database. 3. A DataAdapter object – to provide a mechanism for reading and writing data to the database. 4. A DataSet object – this will contain the table(s) that we will use. The following figure shows how all these pieces tie together. Firstly, we need a connection to the data source, provided by a Connection object. The Connection object requires certain information for it to connect to the data source. The Connection object is called by the DataAdapter object, which handles commands to select, update, insert, and delete data in the data source. Finally we have a DataSet that contains our tables and which uses the DataAdapter to populate itself and to update information in the data source. The User Interface for the Database 5 Basic Data Components DataAdapter Data Connection Data Source DataSet Table1 Table2 We will now cover these components in a bit more detail, taking each of the data source, Connection, DataAdapter and DataSet in turn. The Data Source A data source is the term used to describe any collection of information that can provide data to us. It can take the form of a database, an XML document, a Microsoft Excel spreadsheet, or even a flat text or binary file. It only takes one or two lines of code for us to change the kind of data source that we connect to. The Windows environment provides us a shared set of classes for use in our programs to communicate with these different sources using similar code. The Data Connection The first thing we need to connect to a database is the data Connection object. This comes in two versions – either a SqlConnection or OleDbConnection object. As we are working with the SQL Server Desktop Engine, we will use the SqlConnection object. When we create a connection using a SqlConnection object, we need to feed it the following connection parameters: ❑ Data Source – the name of the server where your data source is located. The data source can be anywhere, be it on your network or somewhere over the Internet. Usually, you will be working on your local network and so you need to specify the name of the computer that holds the data source here. Alternatively, we can give the name localhost or (local) to signify that we want to use the computer that is actually running the application. This terminology is used by many Windows applications when it is necessary to identify the current, local computer. ❑ User ID and Password – the authentication details required to communicate with the data source. The ID and password is set up by the database administrator and helps prevent people from viewing or modifying the database without permission. Chapter 5 6 ❑ Initial Catalog – this is the name of the database we want to work with – in this case, NorthwindSQL. To create a new connection, we declare a new SqlConnection and set the ConnectionString property using these parameters as shown here: Dim myConnection As New SqlClient.SqlConnection() myConnection.ConnectionString = "Data Source=localhost;" & _ "Initial Catalog=NorthwindSQL;User Id=sa;Password=sa;" Alternatively, we can pass the connection string as a parameter to the SqlConnection as follows: Dim myConnection As New SqlClient.SqlConnection("Data Source=localhost;" & _ "Initial Catalog=NorthwindSQL;User Id=sa;Password=sa;"" Creating a new OleDbConnection object is similar, except that we also need a Provider parameter to describe the type of data source that we are connecting to. So why don't we need that parameter with the SqlConnection object? You've got it – because the provider type will always be SQL and, in fact, if you do try to set the Provider parameter for an SqlConnection object, you will get an error. Now we can look at the component that requires a data Connection object to be set up in order to function, namely the DataAdapter. The DataAdapters The DataAdapter is the mechanism that sits between the data source and the DataSet. We have two types of DataAdapters, the SqlDataAdapter, which is used exclusively for SQL Server databases, and the OleDbDataAdapter, which is used for all other data sources and goes through another layer called OLE DB. Consequently, by avoiding the need for this extra layer, the SqlDataAdapter provides much faster access to data. The OleDbDataAdapter can be used to access SQL Server but, as it then goes through the OLE DB layer, you are well advised to stick with the SqlDataAdapter for optimum performance if you don't anticipate using anything other than SQL Server. This applies to our simple database application in this chapter, and so we work strictly with SqlDataAdapter. The DataAdapter allows selecting, updating, deleting, or inserting data in the data source. These methods are accomplished through the use of the SelectCommand, UpdateCommand, InsertCommand, and DeleteCommand properties to set to the database command string required for that particular operation. Each of these properties is an instance of a Command object, whose job it is to execute a SQL statement or stored procedure and return a result set. For a SQL database, the Command object will hold the actual SELECT, UPDATE, INSERT, and DELETE statement required for a given operation, such as "SELECT * FROM Products" or "DELETE FROM Orders WHERE CustomerID='ABC'". The Command object also stores connection information so it may connect to the database to execute the SQL statement that it contains. Note that, in our simplified case, we will only be working with a SelectCommand as we only need to select information from our database for viewing. The User Interface for the Database 7 As we are dealing with a SQL database, we will be working with the SqlCommand object (as opposed to the OleDBCommand object). When we use the DataAdapter Wizards, for each table you work with you will have a corresponding DataAdapter. When we use the Wizards, the DataAdapters are configured specifically for the chosen table such that all of the methods for updating and retrieving information point to that specific table. To re-use the adapter for another table, we have to essentially rebuild the objects that make up the DataAdapter, which means all of the Command objects. The simpler solution is to assign one DataAdapter per table, and this helps keep your code nice and clean and easy to maintain. When you build a DataAdapter, you can specify more than one table if needed. For example, we could create a DataAdapter that links the Customers table and the Orders table – to enable us to view information from both tables using a single DataAdapter, without needing any code to link them. This method of linking multiple tables into a single view doesn't work really well when it comes to updating information, however, as the DataAdapter Wizard isn't able to properly link tables together to cascade updates or deletes, reinforcing the case for using one DataAdapter per table. This diagram shows the basic structure of a DataAdapter: DataAdapter Command Objects SelectCommand InsertCommand UpdateCommand DeleteCommand The DataSet Finally, a DataSet is a container or collection of tables; it can contain one or more tables and is maintained in memory. Relationships between tables are also stored here. The tables it holds contain information such as customer details or product information in the form of records, or rows. A table may consist of thousands of such rows. **Please insert 5555_05_39.bmp Each DataSet can contain many tables Each Table can contain many rows Rows contain data Table Row Row Row Row Table1 Table2 DataSet DataSet Chapter 5 8 One useful illustration is to think of a DataSet as holding details of a book publisher. A technical publisher might publish books in several categories such as .NET, Java, ASP, and C++. Within each category are individual books – so that a .NET category could have books such as Professional VB.NET, Beginning VB.NET, Professional C#, and Introducing .NET. A table could represent each of these categories, and each book in a category would be represented by a row in the appropriate table. Each row holds details for each book – for example, title, price, ISBN number, publishing date, and the number of pages. Each publisher can publish books for several categories Each category can contain several books Book Details: Title = Professional VB.NET Price = $59.99 ISBN = 1861004974 Published = August 2001 Pages = 950 .NET Professional VB.NET Beg. VB.NET Professional C# Introducing .NET Wrox Press .NET Java ASP C++ Publisher Book Catagories There is no limit to the type of information you can store in a DataSet. Now that we have looked at the internals of a DataSet, let's take a look at how we can put one to use in an application. The components shown in the figure below will be demonstrated in our application: Customer Table Components SqlDataAdapter SqlConnection Northwind Database DataSet Customers [...]... point this to your website or directory to read the schema You can also leave this as is and it will work without any problems Again, we will get onto XML and its role in ADO.NET in Chapter 12 33 Chapter 5 The records from the database become available for retrieval when a connection is opened and a SQL statement is executed, as shown here: ' 'SqlSelectCommand1 ' Me.SqlSelectCommand1.CommandText =... from our SQL SELECT statement, as shown below: 27 Chapter 5 Displaying Database Information to the User One thing our wizards didn't do is add code to actually fill the DataSet with the desired information from the database We have to call the Fill method of the DataAdapter to accomplish this Doubleclick on the Get Data button in Visual Studio, and add the following for the click event: Private Sub... like: 28 The User Interface for the Database It's that easy! So what's going on to make this work? 1 From the user interface – our Windows Form – we click the Get Data button, which calls the Fill method of the SqlDataAdapter to tell the database that we want some records 2 Next, the SqlDataAdapter requests a connection to be opened through the SqlConnection object 3 The SqlConnection object uses the... the initial catalog value represents the database we want to communicate with, persist security info states whether or not sensitive security information should be returned in the connection string, user id and password give the user account to connect to the database, and, finally, packet size sets the size of the data blocks for transmissions to and from the database server This size depends on your... retrieve data from the Customers table, and it connects to the database using SqlConnection1 – remember that we need both a DataAdapter and Connection object to get data from a database Binding Data to Controls Most of the controls in the Toolbox are bindable, which means that we can assign a column, or sometimes a group of columns, from a database table to be displayed as their contents As you move... Northwind Database All Rows Try It Out - Adding a Data Bound DataGrid In this section, I'll take you through the process of adding a bound control to our project 1 2 26 Add a DataGrid from the Windows Forms tab in the Toolbox to Form1 of the CustomerApp project Add a Button to the form and change its Name in the Properties window to btnGetData, and the Text property to Get Data The User Interface for the Database. .. if the names in the database don't match your conventions or are not obvious As an example, set the Alias of CompanyName to Company and PostalCode to Zip Notice how the SQL statement changes from "SELECT CustomerID, CompanyName, Address, City, Region, PostalCode" to "SELECT CustomerID, CompanyName AS Company, Address, City, Region, PostalCode AS Zip" 16 The User Interface for the Database We won't be... query as you want, but we just need the Customers table so doubleclick its name; the table should then appear in the window in the background Now click the Close button 14 The User Interface for the Database 13 Next, select the columns to display: CustomerID, CompanyName, Address, City, Region, and PostalCode As you select columns, the SELECT statement will change accordingly Once you have selected all... 1.) Calls Fill method Windows From DataSet 6.) Returns data SqlDataAdapter Customers 5.) Returns data 2.) Request to open connection/ Issues SELECT command SqlConnection 3. ) Opens connection 4.) Returns customer listing Northwind Database Once we have filled our DataSet, the connection is closed and we are free to use the DataSet however we wish In the last example, we displayed the data on the user... Designer generated code " Public Sub New() MyBase.New() 'This call is required by the Windows Form Designer InitializeComponent() 'Add any initialization after the InitializeComponent() call 30 The User Interface for the Database End Sub 'Form overrides dispose to clean up the component list Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean) If disposing Then If Not (components Is Nothing) . = Professional VB .NET Price = $59.99 ISBN = 1861004974 Published = August 2001 Pages = 950 .NET Professional VB .NET Beg. VB .NET Professional C# Introducing .NET Wrox Press .NET Java ASP C++ Publisher. categories such as .NET, Java, ASP, and C++. Within each category are individual books – so that a .NET category could have books such as Professional VB .NET, Beginning VB .NET, Professional C#,. a database. We need four pieces to build our ADO .NET project: 1. A data source – where the actual data is stored, our database. 2. A Connection object – for connecting us to our database. 3.