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After adding a new record to the Authors table
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seen in Figure 127, where I ve added my own details to the table (well, I m writing this book!)
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12422 SQL Commands and Parameters
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Using SQL commands (passed to the database using a Command object) is the most direct and ef cient way of updating a database However, it can take a lot of coding to build the appropriate commands and populate them with data In particular, setting up the parameters for a command, as in the code segment in Listing 124, can be tedious and therefore error-prone
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'These statements create the basic SQL command Dim Sql As String = _ "INSERT INTO Authors(Author, [Year Born])" & _ "VALUES (@Author, @Year);" Dim dbCmd As New OleDbOleDbCommand(Sql, dbConn) 'But we still 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)) Listing 124: Code to insert parameters into a command
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The block of code in Listing 124 is an extract from Listing 123, and is required because of the use of place markers (@Author, @Year) in the SQL command for the
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12 n Databases in Visual Basic NET
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data we wish to insert If we entered the SQL directly into a command line for a database server, and wished to add the same data, the command would be simply:
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INSERT INTO Authors(Author, [Year Born]) VALUES('McMonnies', 1955)
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An alternative method to do this in Visual Basic code would be to build the command string above by joining the contents of the text boxes to the basic command We could de ne a function to return the result of joining all of the necessary fragments of text together Within the function, the StringFormat() method would be ideal for doing the job of concatenating the parameter values and punctuation required
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12423 Coding SQL Parameters
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Private Function AddCommand(ByVal Name As String, _ ByVal YearBorn As Integer) As String Dim SQL As String SQL = "Insert Into Authors(Name, [Year Born]) " SQL &= StringFormat("Values('{0}', {1})", _ Name, YearBornToString) Return SQL End Function Listing 125: Generating a SQL string to insert data
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In Listing 125, note how the Name parameter was enclosed in single quotes (string delimiters in SQL) Generally, we need to exercise care in this type of code to make sure that all the necessary opening and closing marks are put in the right place There is no such requirement for the integer value YearBorn We could now use this to create a much less complex sub for inserting data from text boxes into the table (see Listing 126)
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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 Dim Sql As String = AddCommand(txtAuthorText, _ txtBornText) Dim dbConn As OleDbOleDbConnection dbConn = New OleDbOleDbConnection(CONNSTR) Dim dbCmd As New OleDbOleDbCommand(Sql, dbConn) dbConnOpen() dbCmdExecuteNonQuery() UpdateView() End If End Sub Listing 126: Using the generated SQL text (see Listing 125)
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124 Reading and Writing Data
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Other types of SQL commands (SELECT, DELETE, UPDATE) can be built and deployed using the same techniques However, if we are coding a large and complex database with many tables, the use of these techniques can require you to write a great deal of program code The DataSet class contains a few tricks that can simplify much of this
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Exercise 125
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The SQL DELETE command uses a Where clause to indicate the record(s) to delete Normally this would use the primary key as an identi er, and a typical SQL delete would be of the form: "DELETE * FROM SomeTable WHERE PrimaryKey=1234" Write a sub that will execute the SQL DELETE command on an author, given the author s primary key value was available in the parameter AuID (a Long integer)
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