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Question

我有一张学生表：

• ID NUMBER（8）
• PARENT_ID NUMBER（8）

有数据：

54, 99
50, null
5, 54,
99, 50

如何按此顺序查询？

50 <-- first, because it has no parent, so it's root
99 <-- second, because it's parent is the root item
54 <-- third, because it's parent is the second item
5  <-- forth, because it's parent is the third item

Answer 1

您需要CONNECT BY条款。

请参阅SQL Fiddle。

希望有所帮助。