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The result of the code in Listing 122
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This code results in a table-style view of the entire contents of the Authors table (see Figure 126), including a scroll bar for access to items far down the list One useful feature of this is that if a row of the ListView is selected by clicking on it with the mouse cursor, the SelectedItems() property will include it (SelectedItems() is a collection property) If a single row is selected, SelectedItems(0)Text will return the value in the left-hand column, which for the view shown is the primary key column of the table This will be useful when dealing with related tables (for example, if we wished to add a new item to a table related to this one)
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Exercise 124
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a) b) c)
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Create a sub to display every title in the Biblio database using a
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Amend the sub so that it saves the list of titles to a text le using a
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Amend the sub to add the list of titles to a ListView control
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1242 Gaining read-write access to data
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The DataReader control does not allow changes to be made to data Its purpose is to collect a set of data as rapidly as possible and return it to a program As soon as you want to apply changes to the data, you are left with a choice: whether to work in SQL directly or to use a DataSet The DataSet class allows you to extract an entire set of data from multiple tables, specify relationships between the data in the tables, add new rows, delete rows or edit the existing data, and then write all of the changes back to the database As you might expect, a DataSet object is complex, can contain large amounts of interrelated data and involves quite a bit of programming effort Before we go on to examine the DataSet class, we ll have a look at some much simpler ways of changing the content of a database
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12421 Using SQL Commands directly
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Using the SQL INSERT, DELETE and UPDATE commands, we can make changes to database tables quickly and with minimum use of the system s resources To do this of course, we need knowledge of SQL and this takes time to develop I ll give a short example here (Listing 123) of an INSERT command to add a new record to the Authors table of Biblio, using data picked up from text boxes (txtAuthor, txtBorn) on a form
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Private Sub btnAdd_Click(ByVal sender As SystemObject, _ ByVal e As SystemEventArgs) _ Handles btnAddClick If txtAuthorText = "" Or txtBornText = "" Then MessageBoxShow("Need to enter name and year of birth") Else 'First define the command text Dim Sql As String = _ "INSERT INTO Authors(Author, [Year Born])" & _ "VALUES (@Author, @Year);" 'Declare and create Connection Dim dbConn As OleDbOleDbConnection dbConn = New OleDbOleDbConnection(CONNSTR) 'Declare and create Command object Dim dbCmd As New OleDbOleDbCommand(Sql, dbConn) 'We need to add a parameter for each value inserted 'The Author's name dbCmdParametersAdd("Author", txtAuthorText) ' and year of birth dbCmdParametersAdd("[Year Born]", _ CType(txtBornText, Integer)) 'Open the connection and execute the command dbConnOpen() dbCmdExecuteNonQuery() 'Refresh the ListView so we can see if the insertion 'has worked UpdateView() End If End Sub Listing 123: Inserting a record into a table
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As you can see, inserting a new record can be quite involved However, the programmer has more control over the process, and it represents the most ef cient way of adding data to a database You may be wondering why only two of the elds of the Authors table have been given values (Figure 126 shows it has three elds) The reason for this is that the Au_ID eld of the Authors table in Biblio has been designated as an Identity eld (in Access, it is called an AutoNumber eld) With this type of eld, each new row added gets the next number in the sequence automatically This guarantees that each row of a table will have a different value in this eld, and so it is suitable to use as a primary key eld The results can be
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