检查表中是否存在值;显示消息框

Question

我有一个'invoices'表和发票号码列。

添加新发票时，我想检查它是否已经存在。如果存在，则向用户生成一个MessageBox，表示已经有该号码的记录。

这怎么办？

我使用Visual Basic 2010和SQL Server 2008

Answer 1

尝试是这样的：

Dim SomeInvoiceNumber As String = "" 
Using conn As New SqlClient.SqlConnection("YourConnectionString") 
    Using cmd As SqlClient.SqlCommand = conn.CreateCommand() 

     cmd.CommandText = "SELECT 1 FROM Invoices WHERE InvoiceNumber = @InvNum" 
     cmd.Parameters.AddWithValue("@InvNum", SomeInvoiceNumber) 
     conn.Open() 
     Dim exists As String = cmd.ExecuteScalar().ToString() 

     If exists = "1" Then 
      MessageBox.Show(SomeInvoiceNumber & " exists already.") 
     End If 
    End Using 
End Using