Question

我有两张这样的表：

// questions_and_answers
+----+---------------+------------------------+---------+---------+
| id |    subject    |           body         | related | deleted |
+----+---------------+------------------------+---------+---------+
| 1  | subject1      | question1              | NULL    | 0       |
| 2  |               | answer1                | 1       | 0       |
| 3  | subject2      | question2              | NULL    | 0       |
| 4  |               | answer2                | 3       | 1       |
| 5  |               | answer3                | 3       | 0       |
| 6  | subject3      | question3              | NULL    | 1       |
| 7  |               | answer4                | 6       | 0       |
+----+---------------+------------------------+---------+---------+
-- related column contains either NULL for questions or the id of its question for answers

// viewed_times
+----+-------------+---------+
| id | question_id | user_id |
+----+-------------+---------+
| 1  | 1           | 123     |
| 2  | 1           | 456     |
| 3  | 6           | 123     |
| 4  | 3           | 123     |
| 5  | 6           | 456     |
| 6  | 1           | 789     |
+----+-------------+---------+

我需要搜索body的问题和答案。

EX1：以下是answer4条目的预期结果：

+------------------+--------------+-----------------------+
| question_subject |     body     | total_question_viewed |
+------------------+--------------+-----------------------+
| subject3         | answer4      | 2                     |
+------------------+--------------+-----------------------+

EX2：以下是question1的预期结果：

+------------------+--------------+-----------------------+
| question_subject |     body     | total_question_viewed |
+------------------+--------------+-----------------------+
| subject1         | question1    | 3                     |
+------------------+--------------+-----------------------+

这是我的问题：

SELECT COALESCE(qa2.subject, qa1.subject) question_subject, 
       qa1.body
FROM questions_and_answers qa1
LEFT JOIN questions_and_answers qa2 ON qa1.related = qa2.id
WHERE qa1.body = ":entry"

我当前的查询返回预期结果的前两列。如何计算total_question_viewed值？

重点是，有时我必须加入qa1.id与viewed_times.question_id，有时我必须加入qa2.id等于viewed_times.question_id。我该怎么处理？

Answer 1

您可以使用coalesce加入右栏：

select subject as question_subject,
    body,
    count(*) as total_question_viewed
from questions_and_answers qa
inner join viewed_items v 
    on coalesce(qa.related, qa.id) = v.question_id
where body like '%answer4%'
group by question_subject, body, v.question_id

请参阅live demo here

Answer 2

如果我理解正确，这是另一个JOIN和GROUP BY：

SELECT COALESCE(qa2.subject, qa1.subject) as question_subject, 
       qa1.body,
       COUNT(vt.question_id)
FROM questions_and_answers qa1 LEFT JOIN
     questions_and_answers qa2
     ON qa1.related = qa2.id LEFT JOIN
     viewed_times vt
     ON vt.question_id = qa1.id
WHERE qa1.body = ":entry"
GROUP BY question_subject, qa1.body;

如何在JOIN的ON子句上创建动态列？

2 个答案: