Hello everyone, I'm a stackoverflow novice.
If I have any wrong about asking this question, hope to redress me. :)
我想做一个提问系统,我在MySQL中有两个表。
表'question':存储问题信息。
表 'question_communication' :它存储管理员和用户之间的问题的回复。
这是详细信息表。
question (Table)
- question_id(INT)
- uid(INT)
- category(CHAR)
- description(CHAR)
- submit_time(DATETIME)
和
question_communication (Table)
- question_reply_id(INT)
- question_id(INT)
- uid(ID)
- reply(CHAR)
- time(DATETIME)
- seen(TINYINT) --- Other side has seen the message or not.(0 is not seen, 1 is seen)
我希望查询结果包括:
question_id, uid, category, description, submit_time, seen(=1), seen(=0)
然后我试着写下面的代码,
SELECT T1.question_id, T1.uid, T1.category, T1.description, DATE_FORMAT(T1.submit_time, '%Y/%m/%d %H:%i') submit_time, SUM(T2.seen = 1) seen, SUM(T2.seen = 0) notseen
FROM question T1, question_communication T2
WHERE T1.question_id = T2.question_id
AND T2.uid != (Here is attribute.)
ORDER BY submit_time DESC
但是 WHERE T1.question_id = T2.question_id
这行在一种情况下可能不起作用。
当T1 (question)
有问题时,
它在T2 (question_communication)
中没有任何回复。
所以 T1.question_id = T2.question_id
将导致 SQL JOIN
出乎我的意料。
我的问题总结:
- 如何查询成功结果如下:
question_id, uid, category, description, submit_time, seen(=1), seen(=0)
- 如果在 T1 中有问题,但在 T2 中没有回复。
seen(=1)
和seen(=0)
必须为零
。
谢谢大家:)
@Gordon Linoff 的回答,我添加了 COALESCE() :
SELECT T1.question_id, T1.uid, T1.category, T1.description,
DATE_FORMAT(T1.submit_time, '%Y/%m/%d %H:%i') as submit_time,
COALESCE(SUM(T2.seen = 1), 0) as seen, COALESCE(SUM(T2.seen = 0), 0) as notseen
FROM question T1 LEFT JOIN
question_communication T2
ON T1.question_id = T2.question_id
GROUP BY T1.question_id, T1.uid, T1.category, T1.description,
DATE_FORMAT(T1.submit_time, '%Y/%m/%d %H:%i')
ORDER BY submit_time DESC;
最佳答案
我认为您只需要一个左连接
。事实上,您应该始终使用明确的 join
语法。您的查询还需要一个 group by
。因此,查询看起来像这样:
SELECT T1.question_id, T1.uid, T1.category, T1.description,
DATE_FORMAT(T1.submit_time, '%Y/%m/%d %H:%i') as submit_time,
SUM(T2.seen = 1) as seen, SUM(T2.seen = 0) as notseen
FROM question T1 LEFT JOIN
question_communication T2
ON T1.question_id = T2.question_id
GROUP BY T1.question_id, T1.uid, T1.category, T1.description,
DATE_FORMAT(T1.submit_time, '%Y/%m/%d %H:%i')
ORDER BY submit_time DESC;
我不知道 T2.uid != (这是属性。)
应该做什么。如果你有一个关于T2
的过滤条件,那么把它放在ON
子句中。
关于MySQL: JOIN (A, B) with SUM B,但在某些情况下 B 不存在行,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/28270512/