Question

如果我有三张桌子：

posts
------
postid  date_posted title   content


views
------
viewid  post_id user_id


users
------
userid  username

列出posts尚未查看的所有user_id的正确方法是什么？这个查询是否正确？
SELECT * FROM posts LEFT JOIN views ON postid=post_id WHERE user_id=123 AND postid=NULL;
如何列出所有posts及其＆＃34;查看状态＆＃34;按user_id？
您是否也可以推荐一个优秀的（用户友好的，非geeky）资源来提高我对LEFT JOIN，CROSS JOIN和（正常联接？从帖子，视图中选择*）的理解？

谢谢！

Answer 1

1

SELECT *
FROM posts
WHERE postid NOT IN (
    SELECT post_id 
    FROM views
    WHERE user_id = 123
)

OR

SELECT *
FROM posts
LEFT JOIN views ON views.post_id = posts.postid AND views.user_id = 123
WHERE viewid IS NOT NULL

Answer 2

1.select * from posts where post_id in(select post_id from views where user_id<>123)
or
select * from posts p join views v
on p.post_id=v.post_id
and v.user_id<>123

2.select p.*,CASE when isnull(v.viewid,0)=0 then 0 else 1 end as viewstatus
from posts p left join views v
on p.post_id=v.post_id

Answer 3

在回答您的第3个问题时，请查看visual-explanation-of-sql-joins from codinghorror.com中的链接。这可以帮助您增强连接的基础。

Answer 4

1，您还需要包含users表，但这大致是正确的查询是

2，SELECT * FROM发布LEFT JOIN视图ON postid = post_id应显示它将显示所有帖子

3，http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html

Answer 5

1）见“scoota269”的回答

2）查看您自己的SQL，但删除AND postid=NULL

3）http://www.w3schools.com/sql/sql_join.asp

加入三个mysql表

5 个答案: