如何允许在 TDBLookupComboBox 中选择 NULL 值? [英] How to allow selecting a NULL value in a TDBLookupComboBox?

问题描述

我有一个 TDBLookupComboBox 显示一个 TStringField 类型的 fkLookup，它允许 Null 值(来自可为空的整数数据库列).

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

下拉列表显示分配的 LookupDataSet 中的项目，该项目来自连接表.如果字段为Null，则不显示任何列表项，组合框为空.如果该字段有值，则会显示正确的描述.

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.

我可以通过按分配的 NullValueKey 将其重置为 Null.

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

没关系，但用户更喜欢使用鼠标.所以我可以提供一个清晰的按钮，但我认为列表顶部的附加行会好得多.我该怎么做?

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?

推荐答案

您可以将空行放入查询中，如果需要对其进行排序，可以将其显示在列表顶部，如下所示:

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

sort 列将使其显示在顶部，之后您可以根据需要进行排序.

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

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