Question

我有一个feedback_ques，feedback_ans表，如下所示：

feedback_ques
 
id  question                created_date    created_by delete_date
1   How was the training    16-SEP-20       900         null
2   facility?               16-SEP-20       900         null    
3   Dept?                   16-SEP-20       902         null
4   Infrastructure          16-SEP-20       900         16-SEP-20

feedback_ans

ques_id member_id   answers created_date    created_by
1       10          good    16-SEP-20       891
2       10          good    16-SEP-20       891
3       10          poor    16-SEP-20       891
4       10          good    16-SEP-20       891

我要按如下方式加入表：

select q.id as id, a.member_id as memberId, q.question as Ques, a.answers as Ans
from feedback_ques q, feedback_ans a
where q.id = a.ques_id(+) 
and a.member_id = 10
and q.DELETE_DATE IS NULL;

这给了我所有领域。如果查询表中未找到答案，我希望查询返回null。例如member_id 20没有答案，因此我希望该表在此查询中显示如下的空值。

select q.id as id, a.member_id as memberId, q.question as Ques, a.answers as Ans
from feedback_ques q, feedback_ans a
where q.id = a.ques_id(+) 
and a.member_id = 20
and q.DELETE_DATE IS NULL;

ID  memberId    Ques                    ans
1   20          How was the training    null
2   20          facility?               null
3   20          Dept?                   null
4   20          Infrastructure          null

已更新：如我所建议的那样，使用leftOuter连接如下：

select q.id as id, a.member_id as memberId, q.question as Ques, a.answers as Ans
from feedback_ques q
left join feedback_ans a on a.ques_id = q.id and a.member_id = 20
and q.delete_date is null;

但是，当delete_date为！= null时，此查询不起作用。该行仍由查询返回。由于delete_date！= null，因此不应返回上述任务4。你能帮忙吗？

Answer 1

这些老式的隐式联接使表达您想要的内容变得不容易。这里，where子句中的条件a.member_id = 20过滤掉不匹配的行。

这只是应始终使用显式标准联接的原因之一。考虑一个left join，其中左表上的所有条件都放在on子句中：

select q.id as id, a.member_id as memberId, q.question as Ques, a.answers as Ans
from feedback_ques q
left join feedback_ans a on a.ques_id = q.id and a.member_id = 20
where q.delete_date is null;

如果右侧表没有数据，则联接表以显示空

1 个答案: