Validate SQL data and check for Null - sql

Question

I have a textbox(txtOrderNum) where you enter an 'order' number and update a 'userfield' based on a date that is generated in another textbox(txtDate), I want to validate that the 'order' number entered matches the 'Order' number in the database, then update 'UserField1' only if its NULL, if it doesn't match then do nothing.
My goal, I don't want to overwrite data that is already in the Userfield1, and I want to ensure I update the correct 'Order' number that exist.
****I've updated my code based on suggestion below, I just need to validate if 'Order' number exist AND matches the order number entered in the textbox, then run update query. (also need to parameterized the query, I have some ideas but could use some help)****
Public Sub executequery(ByVal query As String)
Try
Dim cmd As New SqlCommand(query, conn)
conn.Open()
If (conn.State = ConnectionState.Open) Then
cmd.ExecuteNonQuery()
conn.Close()
Else
MessageBox.Show("Check Connection")
conn.Close()
End If
Catch ex As Exception
MsgBox(ex.ToString)
Return
Finally
conn.Close()
End Try
End Sub
'My call Event via Enter Key within Textbox (txtOrdernum)
Dim conn As New SqlConnection("My data source")
Try
Dim updatequery As String = ("UPDATE [DATA].[dbo].[Order] SET [Userfield1] = '" & txtDate.Text.Trim() & "' WHERE [order] ='" & txtOrdernum.Text.Trim() & "' AND [Userfield1] IS NULL")
If e.KeyChar = Chr(13) Then 'Chr(13)
If txtOrdernum.Text.Length >= 8 Then
'MessageBox.Show(updatequery)
executequery(updatequery)
Else
MessageBox.Show("Invalid Order Number'")
End If
Catch ex As Exception
MsgBox(ex.ToString)
Return
End Try

Answer 1

As N0Alias stated, add "AND [UserField1] IS NULL".
Just be careful with the way you build your query. Building one like your example can allow SQL injection.
You should use the 'SqlCommand.Parameters' property to add values into your query.