Question

我有一个像SO这样的问答网站。我有一张这样的桌子：

// qanda
+----+----------+----------------------+---------+------+
| id |   title  |       content        | related | type |
+----+----------+----------------------+---------+------+
| 1  | a title  | a content            | NULL    | 0    |
| 2  |          | a content            | 1       | 1    |
| 3  | a title  | a content            | NULL    | 0    |
| 4  |          | a content            | 1       | 1    |
| 5  |          | a content            | 3       | 1    |
+----+----------+----------------------+---------+------+
/*  type column:     "0" means it is a question, "1" means it is a answer
    related column:  it contains the id number of its own question
*/

现在我需要根据id号码选择一个问题和所有答案。 （id号码可以是问题的ID或答案的ID）。

这是我当前的查询：

SELECT * FROM qanda WHERE id = :id OR related = :id

仅当:id是问题的ID时，我的查询才有效。 （我的意思是如果:id是答案的ID，它无法正常工作）。

以下是预期结果：

assuming either :id = 1 or :id = 2 or :id = 4
+----+----------+----------------------+---------+------+
| id |   title  |       content        | related | type |
+----+----------+----------------------+---------+------+
| 1  | a title  | a content            | NULL    | 0    |
| 2  |          | a content            | 1       | 1    |
| 4  |          | a content            | 1       | 1    |
+----+----------+----------------------+---------+------+

正如我上面提到的，如果:id = 1或:id = 2或:id = 4，我需要选择这三行。我怎么能这样做？

Answer 1

以下查询应该有效。查询分为4个部分，它们组合在一起。每个查询的描述：

如果:id是一个问题，则返回问题
如果:id是一个问题
如果:id是答案，则返回问题
如果:id是答案，则返回答案

查询：

select q.*
  from quanda q
 where q.id = :id
   and q.type = 0
 UNION ALL
select a.*
  from quanda a
 where a.related = :id
 UNION ALL
select q.*
  from quanda a
  join quanda q
    on q.id = a.related
 where a.id = :id
   and a.type = 1 
 UNION ALL
select a2.*
  from quanda a1
  join quanda a2
    on a2.related = a1.related
 where a1.id = :id
   and a1.type = 1

如何选择帖子及其所有相关帖子？

1 个答案: