Question

我一直试图解决这个问题一段时间，希望有人帮助我。我有两张桌子，第一张桌子是

表名：OnlineTest

OnlineTestId     category    subcategory                                                                   
     1            English      Spelling                                                                    
     2            English      Grammar
     3            English      Antonyms
     4            English      Synonyms

第二个表是

表名：UserStatus

Id     userId    status         onlineTestId
1       1        Finished           1
2       1        Not Finished       2
3       2        Not Finished       1
4       2        Finished           3
5       3        Not Finished       4

结果

OnlineTestId    userId        status
    1               1         Finished
    2               1         Not Finished
    3               null      null
    4               null      null

我试过这个查询，

select c.onlinetestid, d.userid, d.status from onlinetest c left join userstatus d on d.onlinetestid = c.onlinetestid
where c.category = 'English' and d.userid = 1;

但是这个查询带来了结果的前两行而不是最后两行，其中userId和status为null。

如何得出上述结果？

Answer 1

将d.userid = 1谓词放在ON子句中：

select c.onlinetestid, d.userid, d.status 
from onlinetest c 
left join userstatus d on d.onlinetestid = c.onlinetestid and d.userid = 1
where c.category = 'English'

这将返回onlinetest的所有行，userstatus列填充null s，其中谓词d.userid = 1失败。

Answer 2

您也可以使用左外连接，如下所示：

SELECT        c.OnlineTestId, d.userId, d.status
FROM            OnlineTest AS c LEFT OUTER JOIN
                         UserStatus AS d ON d.onlineTestId = c.OnlineTestId AND d.userId = 1
WHERE        (c.category = 'English')

Mysql左边的连接条件在右表中

2 个答案: