什么是查询显示2表中的内容没有重复的数据？

Question

我有两个表如下

forum_t (forum_id PK, question,description). 

forum_answer (forum_answer_id PK,comment,forum_id). 

现在我想通过这两个表合并，以获得问题，说明和（特定forum_id评论这是第一台和第二表的外键的主键结果）。

Answer 1

SELECT question,description,comment 
FROM 
    forum_t 
    INNER JOIN 
    forum_answer 
    ON forum_t.forum_id=forum_answer.forum_id 
WHERE 
    forum_t.forum_id={your forum id} 

Answer 2

要删除重复项，使用DISTINCT关键字，但你不会得到每每个问题的一条记录，你可以有很多答案和评论。

SELECT DISTINCT F.forum_id,F.question,F.description,A.comment 
FROM forum_t F INNER JOIN forum_answer A on F.forum_id = A.forum_id 
WHERE F.forum_id = 1 

Answer 3

如果你真的想每个问题只有一行，那么你将不得不使用某种聚集函数。

首先计数（每个问题的答案数）。

select q.forum_id, q.question, q.description, count(*) as answer_count 
from forum_t q 
inner join forum_answer a on (q.forum_id = a.forum_id) 
where q.forum_id between 1 and 10 -- first 10 questions 
group by q.forum_id 

如果你想实际的答案，你可以使用GROUP_CONCAT放一起，然后

select 
    q.forum_id 
    , q.question 
    , q.description 
    , count(*) as answer_count 
    , group_concat(a.comment) as comments_in_csv_form 
from forum_t q 
inner join forum_answer a on (q.forum_id = a.forum_id) 
where q.forum_id between 1 and 10 -- first 10 questions 
group by q.forum_id 

您现在可以使用PHP或任何的langauge您使用客户方爆炸的意见。

参见：http://dev.mysql.com/doc/refman/5.5/en/group-by-functions.html#function_group-concat