Question

我有两个表：qanda和comment。以下是它们的结构：

// 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
*/

// comment
+----+---------+---------+-----------------+
| id | post_id | user_id |     content     |
+----+---------+---------+-----------------+
| 1  | 1       | 324523  | a content       |
| 2  | 5       | 435243  | a content       |
+----+---------+---------+-----------------+

我在id表中只有一个comment。这是我目前的查询：

SELECT post_id FROM comment WHERE id = :id

当前输出：

// assuming :id = 2
+---------+
| post_id |
+---------+
| 5       |
+---------+

但我还需要选择自己问题的ID。所以这是预期结果：

// assuming :id = 2
+-------------+---------+
| question_id | post_id |
+-------------+---------+
| 3           | 5       |
+-------------+---------+

那我怎么能这样做？

Answer 1

如果我理解正确，你只需要join这些表：

SELECT c.post_id, q.related
FROM comment c
    join qanda q on c.post_id = q.id
WHERE c.id = :id

A Visual Explanation of SQL Joins

Answer 2

假设qanda表中的id是post_id且相关的是question_id，则此查询应该完成工作：

SELECT qanda.related AS question_id, qanda.id AS post_id
FROM qanda, comment
WHERE comment.id = id AND qanda.id = comment.post_id

如何从另一个表中获取ID号？

2 个答案: