i want to trap, example i have foreign key or itemno 1 with rose if i attemp to add rose under itemno 1 it will trap me

Code:
If TxtItemNu.Text = "" Then
           MsgBox "No Item Number entered.", vbOKOnly + vbCritical, "ERROR: Empty field/s. "
'           TxtItemNu.SetFocus
           Exit Sub
       End If
       
If TxtFlowerID.Text = "" Then
           MsgBox "No Flower ID entered.", vbOKOnly + vbCritical, "ERROR: Empty field/s. "
'           TxtFlowerID.SetFocus
           Exit Sub
       End If
       
If CboQuantity.Text = "" Then
           MsgBox "No Quantity entered.", vbOKOnly + vbCritical, "ERROR: Empty field/s. "
           CboQuantity.SetFocus
           Exit Sub
       End If
       
     On Error GoTo errSave
        Dim sqlstr As String
            sqlstr = "Insert Into STRUCTURE(STR_ITEMNO,STR_FLOWER,STR_QTY) values(" & TxtItemNu.Text & "," & TxtFlowerID.Text & "," & CboQuantity.Text & ")"
           
                 If CStr(TxtItemNu.Text) = CStr(TxtFlowerID.Text) Then
                MsgBox ("The Item Number Can't be The Same with Flower ID")
                    Exit Sub
                End If
            
            Conn.BeginTrans
            Conn.Execute sqlstr
            Conn.CommitTrans
            
            MsgBox "STRUCTURE successfully saved!"
            
            CmdSave.Enabled = False
            CmdDelete.Enabled = False
            
            ClearText
            Unload Me
            Form1.Show
        
ExitErr:
    Exit Sub
errSave:
        Conn.RollbackTrans
        MsgBox "Error" & Err.Description, vbCritical
Resume ExitErr
                  
End Sub
how can i do that?
to avoid redundancy itemname in every itemno...