如果右側表沒有數據，則連接表以顯示空值

Question

我有一個feedback_ques，feedback_ans表如下：

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

我想按如下方式加入表格：

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;

這給了我所有的領域。 如果在答案表中找不到答案，我希望查詢為答案返回 null。 例如，member_id 20 沒有答案，因此我希望該表為該查詢顯示如下空值。

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

更新：按照我的建議使用 leftOuter join 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;

但是當 delete_date 為 !=null 時，此查詢不起作用。 該行仍由查詢返回。 從上面的任務 4 不應該返回，因為 delete_date != null。 你能幫忙嗎。

Answer 1

這些陳舊的、隱式的連接讓你很難表達你想要的東西。 這里，where 子句中的條件a.member_id = 20過濾掉了不匹配的行。

這只是應始終使用顯式標准連接的原因之一。 考慮一個left join ，其中左表上的所有條件都放在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;