How to allow selecting a NULL value in a TDBLookupComboBox?

Question

I have a TDBLookupComboBox showing a TStringField of kind fkLookup, which allows Null values (from a nullable Integer database column).

The dropdown list displays the items from the assigned LookupDataSet, which comes from a joined table. If the field is Null, none of the list items are displayed, the combobox is empty. If the field has a value, the correct description is shown.

I can reset it to Null by pressing the assigned NullValueKey.

That's all ok, but the users prefer using the mouse. So I could provide a clear button, but I think an additional row on top of the list would be much better. How can I do that?

Answer 1

You can put the empty row in your query, and if you need it sorted you can make it appear at the top in your list like this:

select 0 as sort,
       convert(int, null) as UserID,
       'Clear' as Name
union all
select 1 as sort,
       u.UserID,
       u.Name
from   tblUser u
order by sort, Name

The sort column will make it appear at the top, after that you can sort on whatever you need.