dblookupcombobox有一个空行 [英] dblookupcombobox has a null row

问题描述

我有一个关于DBLookupComboBox的问题。

I have a question about DBLookupComboBox.

我有一个程序，我写了一个数据库。它有一切，除非我打开DBLookupComboBox它必须有一个空值的行，当用户想要选择什么。但是没有一个。

I have a program that has a database I wrote. It has everything, except when I open DBLookupComboBox it must have a row with null value for when user wants to select nothing. But there isn't one. How can I make a null row show up?

推荐答案

您必须添加一行Nothing或All ，永远适合。通常的解决方案是UNION查询，它可以用作组合框的RowSource。联合查询可用于添加虚拟字段。

You must either add a row that says 'Nothing' or 'All', which ever suits. The usual solution is a UNION query, which can be used as the RowSource of the combobox. The Union query can be used to add virtual fields.

如果组合仅包含唯一值，您可以说：

If the combo comtains only unique values, you can say:

SELECT "Nothing" As Description
FROM ATable
UNION 
SELECT Description
FROM ATable

UNION消除重复项，UNION ALL <没有，所以如果有匹配的行，你可以说：

UNION eliminates duplicates, UNION ALL< does not, so if there are matching rows, you can say:

SELECT DISTINCT "Nothing" As Description
FROM ATable
UNION ALL
SELECT Description
FROM ATable

想要无排序，你必须拼凑一点，使用无或无，但如果你有一个ID或键列，你可以得到一个很好的排序，如：

If you want "Nothing" to sort first, you must juggle a little and use " Nothing", or "-Nothing", but if you have an ID or Key column you can get a nice sort, like so:

SELECT 0 As ID, "Nothing" As Description
FROM ATable
UNION 
SELECT ID, Description
FROM ATable

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