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CHAPTER 6 s INTRODUCING COMMANDS
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' Execute query to get number of employees Console.WriteLine( _ "Before INSERT: Number of employees = {0}", cmdqry.ExecuteScalar()) ' Execute nonquery to insert an employee cmdnon.Parameters("@fname").Value = fname cmdnon.Parameters("@lname").Value = lname Console.WriteLine( _ "Executing statement " & cmdnon.CommandText) cmdnon.ExecuteNonQuery() Console.WriteLine( _ "After INSERT: Number of employees = " & cmdqry.ExecuteScalar()) ' Execute nonquery to delete an employee cmdnon.CommandText = sqldel Console.WriteLine( _ "Executing statement " & cmdnon.CommandText) cmdnon.ExecuteNonQuery() Console.WriteLine( _ "After DELETE: Number of employees = " & cmdqry.ExecuteScalar()) Catch e As Exception ' Display error Console.WriteLine("Error: " & e.ToString) Finally ' Close connection conn.Close() End Try End Sub End Module
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3. Make it the startup project, and then run it with Ctrl+F5. You should see the result in Figure 6-7.
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Figure 6-7. Using command parameters
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First, you set up sample data:
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' Set up rudimentary data Dim fname As String = "Zachariah" Dim lname As String = "Zinn"
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then you added two parameters, @fname and @lname, to the DML:
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' Set up insert statement Dim sqlins As String = _ "insert into employees " _ & "( " _ & " firstname, " _ & " lastname " _ & ") " _ & "values('@fname', '@lname')" ' Set up delete statement Dim sqldel As String = _ "delete from employees " _ & "where " _ & " firstname = '@fname' " _ & " and " _ & " lastname = '@lname' "
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and also to the Parameters collection property of the command you wanted to parameterize:
' Add parameters to the command for executing statements cmdnon.Parameters.Add("@fname", SqlDbType.NVarChar, 10) cmdnon.Parameters.Add("@lname", SqlDbType.NVarChar, 20)
Note that you provided the parameter names as strings and then specified the data types of the columns you expected to use them with. The SqlDbType enumeration contains a member for every SQL Server data type except cursor and table, which can t be directly used by Visual Basic programs. The Add method is overloaded. Since nvarchar requires us to specify its maximum length, you included that as the third argument. Finally, you set the parameter values before executing the command:
' Execute nonquery to insert an employee cmdnon.Parameters("@fname").Value = fname cmdnon.Parameters("@lname").Value = lname
CHAPTER 6 s INTRODUCING COMMANDS
s Note You used the same command, cmdnon, to execute both the INSERT and DELETE statements. The parameter values didn t change, even though the SQL in CommandText did. The Parameters collection is the source of parameter values for whatever SQL is in CommandText. The SQL does not have to use all or even any of the parameters, but it cannot use any parameters not in the command s Parameters collection.
Notice in Figure 6-7 that when you displayed the SQL in CommandText you saw the parameter names rather than their values. Values are substituted for parameters when the SQL is submitted to the database server, not when the values are assigned to the members of the Parameters collection.
The Prepare Method
When you expect to execute a parameterized command multiple times, you should prepare it with the Prepare method. The syntax is simple:
command.Prepare()
and it can be executed any time after the parameters have been added to the command and before the command is executed. Prepare() avoids SQL parsing overhead. The query or statement associated with the command is parsed only on first execution. After that, the cached SQL is executed, changing only parameter values. You never have to prepare any commands, but it s always the best practice to do this if you expect to execute a command multiple times.
s Note If you change its CommandText after you prepare a command, you must prepare the command
again to gain the advantage of prepared SQL. Only parameter values can change between command executions, for a command to stay prepared.
You can use Prepare() even if you only execute a command once, but it s a waste of your time and the computer s. For example, you could change Module1.vb as follows:
' Execute nonquery to insert an employee cmdnon.Parameters("@fname").Value = fname cmdnon.Parameters("@lname").Value = lname Console.WriteLine( _ "Executing statement " & cmdnon.CommandText) cmdnon.ExecuteNonQuery()
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