[英]SQL Query joining two tables similar to left outer join
表 1:问题
id | question
----------------
1 | Name
2 | Age
3 | Gender
4 | Position
表 2:答案
qid | ans | record
-----------------------
1 | Jay | 1
2 | 24 | 1
3 | M | 1
2 | 23 | 2
我想提出一个结果如下表的连接查询:
record | question | ans
-----------------------------
1 | Name | Jay
1 | Age | 24
1 | Gender | M
1 | Position | null
2 | Name | null
2 | Age | 23
2 | Gender | null
2 | Position | null
我能想到的最接近的是这个连接:
select a.record, q.question, a.ans
from
question q left outer join answer a
on q.id = a.qid order by a.record,q.id;
然而,这个查询只产生这个,但我希望所有的问题都显示两次
record | question | ans
-----------------------------
1 | Name | Jay
1 | Age | 24
1 | Gender | M
1 | Position | null
2 | Age | 23
您需要一个交叉连接来生成您需要的所有组合,并与左连接配对以检索答案,如下所示:
select
r.record,
q.question,
a.ans
from question q
cross join (select distinct record from answer) r
left join answer a on a.record = r.record and a.qid = q.id
order by r.record, q.id
结果:
record question ans
------ -------- ------
1 Name Jay
1 Age 24
1 Gender M
1 Position <null>
2 Name <null>
2 Age 23
2 Gender <null>
2 Position <null>
作为参考,这是我用来验证案例的测试脚本:
create table question (
id int,
question varchar(10)
);
insert into question (id, question) values
(1, 'Name'),
(2, 'Age'),
(3, 'Gender'),
(4, 'Position');
create table answer (
qid int,
ans varchar(10),
record int
);
insert into answer (qid, ans, record) values
(1, 'Jay', 1),
(2, '24', 1),
(3, 'M', 1),
(2, '23', 2);
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