表有2个字段，但一次只能使用一个

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如何设计一个包含2个字段的表的数据库，其中只有一个字段可以一次设置，没有太多的冗余？例如文件系统：

How would you design a database that has a table with 2 fields and just one of them can be set at a time, without too much redundancy? For example a file system:

假设我们有一个带有驱动器的表，一个带有文件夹，一个带有文件。

Let's say that we have a table with drives, one with folders and one with files.

驱动器和文件非常简单。但是，文件夹有一个父文件夹，可以是一个文件夹（在这种情况下，引用到同一个表）或一个磁盘（在这种情况下，引用到一个磁盘行）。

Drives and files are quite trivial. However, folders have a parent which can be either a folder (in which case the reference is to the same table) or a disk (in which case the reference is to a disk row).

您会添加一些额外的表吗？

Would you add some extra tables?

推荐答案

一个表，id，name，parentid，想要）类型。文件夹和文件都使用parentid指向其父级。磁盘没有父项。只有NULL的字段是类型为Disk的（相对较少）记录的parentid字段。

One table with id, name, parentid, and (if you want) type. Folders and files all point to their parent with parentid. Disks do not have a parentid. The only field that is NULL is the parentid field of the (relatively few) records that are of type Disk.