SQL 三表联查

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因需要在三个表中查东西，还不能要交集，要并集。

创建三张表

我用的是DBaver来创建，下面就是代码，跑完之后会出现三个表。

create table tas_master.hk_test_1(
	id INT not null,
	name1 varchar(100) NULL
);

create table tas_master.hk_test_2(
	id INT not null,
	name1 varchar(100) null,
	name2 varchar(100) NULL
);

create table tas_master.hk_test_3(
	id INT not null,
	name1 varchar(100) null,
	name3 varchar(100) NULL
);


insert into tas_master.hk_test_1 
(id, name1) 
values 
(1, 's1'),
(2, 's2'),
(3, 's3');


insert into tas_master.hk_test_2 
(id, name1, name2) 
values 
(2, 's2', '2s2'),
(3, 's3', '3s3'),
(4, 's4', '4s4');


insert into tas_master.hk_test_3 
(id, name1, name3) 
values 
(1, 's1', '1l1'),
(1, 'ss1', '1l1'),
(2, 's2', '2l2'),
(3, 's3', '3l3'),
(4, 's4', '4l4'),
(5, 's5', '5l5');

select * from tas_master.hk_test_1;
select * from tas_master.hk_test_2;
select * from tas_master.hk_test_3;

第一个表tas_master.hk_test_1

id	name1
2	s2
3	s3
1	s1

第二个表tas_master.hk_test_2

id	name1	name2
2	s2	2s2
3	s3	3s3
4	s4	4s4

第三个表tas_master.hk_test_3

id	name1	name3
3	s3	3l3
2	s2	2l2
5	s5	5l5
1	s1	1l1
1	ss1	1l1
4	s4	4l4

来三表联查一下吧

就是选用三个表的数据，一定是满足on条件的。

inner join 内连接

select coalesce(t1.id, t2.id, t3.id) as id, t1.name1, t2.name1, t2.name2, t3.name1, t3.name3 from 
(select * from tas_master.hk_test_1) as t1 
join (select * from tas_master.hk_test_2) as t2 
on (t1.id = t2.id and t1.name1 = t2.name1) 
join (select * from tas_master.hk_test_3) as t3
on (t2.id = t3.id and t3.name1 = t2.name1);

结果，可看作交集

id	name1	name1	name2	name1	name3
3	s3	s3	3s3	s3	3l3
2	s2	s2	2s2	s2	2l2

full join 全连接

select coalesce(t1.id, t2.id, t3.id) as id, t1.name1, t2.name1, t2.name2, t3.name1, t3.name3 from 
(select * from tas_master.hk_test_1) as t1 
full join (select * from tas_master.hk_test_2) as t2 
on (t1.id = t2.id) 
full join (select * from tas_master.hk_test_3) as t3
on (t1.id = t3.id);

结果，可看作并集，注意on后面只能跟一个条件，多了不好用

id	name1	name1	name2	name1	name3
2	s2	s2	2s2	s2	2l2
1	s1			ss1	1l1
1	s1			s1	1l1
5				s5	5l5
4		s4	4s4
4				s4	4l4
3	s3	s3	3s3	s3	3l3

我们看下用不同的on后面条件，出来结果是否一样。

insert into tas_master.hk_test_3 
(id, name1, name3) 
values 
(6, 's1', '1l2');

tas_master.hk_test_3 就变成这样，多了id_6

id	name1	name3
3	s3	3l3
6	s1	1l2
5	s5	5l5
2	s2	2l2
1	s1	1l1
1	ss1	1l1
4	s4	4l4

name1条件

select coalesce(t1.id, t2.id, t3.id) as id, t1.name1, t2.name1, t2.name2, t3.name1, t3.name3 from 
(select * from tas_master.hk_test_1) as t1 
full join (select * from tas_master.hk_test_2) as t2 
on (t1.name1 = t2.name1) 
full join (select * from tas_master.hk_test_3) as t3
on (t1.name1 = t3.name1);

id	name1	name1	name2	name1	name3
3	s3	s3	3s3	s3	3l3
1				ss1	1l1
1	s1			s1	1l2
1	s1			s1	1l1
5				s5	5l5
4		s4	4s4
4				s4	4l4
2	s2	s2	2s2	s2	2l2

id条件

select coalesce(t1.id, t2.id, t3.id) as id, t1.name1, t2.name1, t2.name2, t3.name1, t3.name3 from 
(select * from tas_master.hk_test_1) as t1 
full join (select * from tas_master.hk_test_2) as t2 
on (t1.id = t2.id) 
full join (select * from tas_master.hk_test_3) as t3
on (t1.id = t3.id);

id	name1	name1	name2	name1	name3
6				s1	1l2
4		s4	4s4
4				s4	4l4
3	s3	s3	3s3	s3	3l3
2	s2	s2	2s2	s2	2l2
5				s5	5l5
1	s1			ss1	1l1
1	s1			s1	1l1

两个还不是一样的，我认为只要on条件中，筛出来的那一列肯定是有值的。
我有尝试了不同的条件组合，发现这个on条件中还是只放一个为好，下面是我做的记录。

select t1.id, t2.id, t3.id, t1.name1, t2.name1, t2.name2, t3.name1, t3.name3 from 
(select * from tas_master.hk_test_1) as t1 
full join (select * from tas_master.hk_test_2) as t2 
on (t1.id = t2.id) 
full join (select * from tas_master.hk_test_3) as t3
on (t2.id = t3.id and t1.id = t3.id and t1.id = t2.id);

id	id	id	name1	name1	name2	name1	name3
3	3	3	s3	s3	3s3	s3	3l3
	6				s1	1l2
1			s1
	1				ss1	1l1
	1				s1	1l1
2	2	2	s2	s2	2s2	s2	2l2
4			s4	4s4
	4				s4	4l4
	5				s5	5l5

select t1.id, t2.id, t3.id, t1.name1, t2.name1, t2.name2, t3.name1, t3.name3 from 
(select * from tas_master.hk_test_1) as t1 
full join (select * from tas_master.hk_test_2) as t2 
on (t1.id = t2.id) 
full join (select * from tas_master.hk_test_3) as t3
on (t2.id = t3.id and t1.id = t3.id);

id	id	id	name1	name1	name2	name1	name3
3	3	3	s3	s3	3s3	s3	3l3
1			s1
	1				ss1	1l1
	1				s1	1l1
	6				s1	1l2
	5				s5	5l5
2	2	2	s2	s2	2s2	s2	2l2
4			s4	4s4
	4				s4	4l4

select t1.id, t2.id, t3.id, t1.name1, t2.name1, t2.name2, t3.name1, t3.name3 from 
(select * from tas_master.hk_test_1) as t1 
full join (select * from tas_master.hk_test_2) as t2 
on (t1.id = t2.id) 
full join (select * from tas_master.hk_test_3) as t3
on (t2.id = t3.id);

id	id	id	name1	name1	name2	name1	name3
3	3	3	s3	s3	3s3	s3	3l3
	5				s5	5l5
4	4		s4	4s4	s4	4l4
2	2	2	s2	s2	2s2	s2	2l2
1			s1
	1				ss1	1l1
	1				s1	1l1
	6				s1	1l2

left join 左连接

这个就简单了就是以左边的为准。

select t1.id, t2.id, t3.id, t1.name1, t2.name1, t2.name2, t3.name1, t3.name3 from 
(select * from tas_master.hk_test_1) as t1 
left join (select * from tas_master.hk_test_2) as t2 
on (t1.id = t2.id) 
left join (select * from tas_master.hk_test_3) as t3
on (t1.id = t3.id);

id	id	id	name1	name1	name2	name1	name3
1		1	s1			ss1	1l1
1		1	s1			s1	1l1
3	3	3	s3	s3	3s3	s3	3l3
2	2	2	s2	s2	2s2	s2	2l2

select t1.id, t2.id, t3.id, t1.name1, t2.name1, t2.name2, t3.name1, t3.name3 from 
(select * from tas_master.hk_test_1) as t1 
left join (select * from tas_master.hk_test_2) as t2 
on (t1.id = t2.id) 
left join (select * from tas_master.hk_test_3) as t3
on (t3.id = t2.id);

id	id	id	name1	name1	name2	name1	name3
1			s1
3	3	3	s3	s3	3s3	s3	3l3
2	2	2	s2	s2	2s2	s2	2l2

给我的感觉是，三表联查，主要是看on条件中是什么样的。想三表联查还是with写好点。