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If CType(ExecuteCommand(updSQL), Integer) = 0 Then MessageBoxShow("Insert Title did not work") End If Dim A As Integer For Each A In mvarAuthors updSQL = StringFormat( _ "Insert Into [Title Author] " & _ "(ISBN, Au_ID) " & _ "Values('{0}', {1})", ISBN, A) If CType(ExecuteCommand(updSQL), Integer) _ = 0 Then MessageBoxShow("Insert Author #" & _ AToString() & " did not work") End If Next ElseIf mvarChanged Then 'Update data fields in existing record updSQL = StringFormat( _ "Update Titles Set Title='{0}', " & _ "[Year Published]={1}, PubID={2} " & _ "Where ISBN='{3}'", mvarTitle, _ mvarYearPublished, mvarPubID, mvarISBN) If CType(ExecuteCommand(updSQL), Integer) = 0 Then MessageBoxShow("Update command did not work") End If End If End Class Listing 1218: The Book class, incorporating database persistence
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The proper working of the Book class relies on two external functions (they are not part of the class because they will be of use to any class that uses the database in a similar way eg Authors and Publishers) GetReader() is a function that takes a SQL Select string and returns a data reader with the corresponding data from database tables We can retrieve all of the information from a row of the Titles table with the SQL query:
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Select * From Titles Where ISBN = <some ISBN string>
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The GetReader() function creates and opens a database connection and retrieves the DataReader so that individual columns can be extracted from it The ExecuteCommand() function is used to execute a SQL command on the database Typically this will be used for non-query commands (INSERTS, DELETES and UPDATES), although the function can also execute a SQL Select command and return a result Two different ways are used to execute the command, depending on whether it includes the word Select or not If there is no Select part to a command, the ExecuteNonQuery() method is used since this will return a count of the number of rows in the database affected by the query If there is a Select part to the command, it should be sent by the ExecuteScalar() method, which
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125 Data Object Modelling
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can return a single item of data This is useful if we are executing an INSERT command where there is an identity eld in the database, since an added Select statement (shown later) can return this The result (if the SQL string is properly constructed) is an update to the database and a single Object result, which must be cast to a type that matches the type of data you expect to be returned GetReader() and ExecuteCommand() are as shown in Listing 1219
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Private Private Private Private CONNSTR = "<usual connection string stuff>" dbConn As SqlClientSqlConnection dbCmd As SqlClientSqlCommand dbRdr As SqlClientSqlDataReader
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Function GetReader(ByVal SQL As String) _ As SqlClientSqlDataReader Dim rd As SqlClientSqlDataReader dbConn = New SqlClientSqlConnection(CONNSTR) dbCmd = New SqlClientSqlCommand(SQL, dbConn) dbConnOpen() rd = dbCmdExecuteReader( _ CommandBehaviorCloseConnection) Return rd End Function Function ExecuteCommand(ByVal SQL As String) As Object Dim result As Integer dbConn = New SqlClientSqlConnection(CONNSTR) dbCmd = New SqlClientSqlCommand(SQL, dbConn) dbConnOpen() If InStr(SQL, "Select", CompareMethodText) > 0 Then result = CType(dbCmdExecuteScalar(), Integer) Else result = dbCmdExecuteNonQuery() End If dbConnClose() Return result End Function Listing 1219: Functions for fetching and storing data
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The Book class has two constructors The rst takes an ISBN string and uses it to retrieve the corresponding book s record I ve omitted any error-handling code for clarity but there should certainly be some, at the very least to indicate where a supplied ISBN string is not in the database Code in this constructor would be very simple but for the need to retrieve a possible list of Author identi cations from the [Title Author] table Note the use of the AddAuthor() method to append the author ids to an ArrayList of authors The second constructor is simpler, since it just takes the supplied parameter data and assigns it to the various member variables Note that author ids are not included in this list since we would never know how many to deal with (I ve seen some computing books with more than a dozen authors names on the jacket) Each author is added individually using the AddAuthor() method
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