Question

这是我的表结构：

-- qanda (stands for questions and answers)
+----+---------+-----------------------------------------------+--------------+
| id |  title  |                      content                  |  question_id |
+----+---------+-----------------------------------------------+--------------+
| 1  | title1  | this is a question                            | NULL         |
| 2  | NULL    | this is an answer                             | 1            |
| 3  | NULL    | this is another answer                        | 1            |
| 4  | title2  | this is another question                      | NULL         |
| 5  | NULL    | this is an answer for the second question     | 4            |
| 6  | NULL    | this is another answer for the first question | 1            |
+----+---------+-----------------------------------------------+--------------+

我知道，如果我将问题和答案放在两个不同的表格中会更好。但现在我只想了解JOIN在这种情况下的确切运作方式。

我有一个qanda表的ID，我总是想要一个标题。该id可能是问题的id或答案的id。我怎么能这样做？

我想要这样的事情：

SELECT t1.title
FROM qanda t1
INNER JOIN qanda t2
ON t1.id = t2.question_id
WHERE t1.id = :id

我的查询没有任何匹配。以下是一些预期结果的样本：

-- :id = 1
+--------+
| title1 |
+--------+

-- :id = 2
+--------+
| title1 |
+--------+

-- :id = 4
+--------+
| title2 |
+--------+

-- :id = 5
+--------+
| title2 |
+--------+

-- :id = 6
+--------+
| title1 |
+--------+

Answer 1

2个查询联盟

SELECT t1.title
FROM qanda t1
WHERE t1.id = :id and t1.title IS NOT NULL
UNION
SELECT t1.Title
FROM qanda t2
JOIN qanda t1
ON t1.id = t2.question_id
WHERE t2.id = :id

可选地

SELECT DISTINCT t1.title
FROM qanda t1
JOIN qanda t2
ON t1.id = t2.question_id
WHERE :id in ( t2.id, t1.id)

Answer 2

与Serg相似;但如果您遇到这种情况，使用左连接将允许带有out（w / o）答案的问题出现在结果中。

SELECT distinct coalesce(t2.title, t1.title) as title
FROM qanda t1 
LEFT JOIN qanda t2 
  ON t1.id = t2.question_id 
WHERE (t1.id = 1 or T2.ID = 1)
  and Type = 0;

如果我们可以假设一个标题只存在于问题上，并且没有答案会有标题。

我认为这很难维护，它应该更快，因为它消除了连接（有点存在可以提前逃脱连接无法实现，并且由于限制发生在子查询上我们只有1记录真的要处理加入）和不同的。

SELECT t1.title as title
FROM qanda t1 
WHERE (EXISTS (SELECT 1 
               FROM qanda t2 
               WHERE ID = 1 
                 and t1.ID = t2.question_id)  --correlated subquery
           or t1.id = 1)
  and Type = 0

Answer 3

您可以尝试这样的事情：

SELECT title 
FROM qanda 
INNER JOIN (
    SELECT DISTINCT COALESCE(t1.question_id, t1.id) AS ID
    FROM qanda t1   
    WHERE :id IN(t1.question_id=:id, t1.id)
    ) B ON qanda.id = B.ID;

我如何加入null？

3 个答案: