多有联合查询

[小草旁的大树]

表A
a_id b_id c_id
1 1 1
2 null 1
3 1 null
4 null null

表B
b_id type1
1 BB

表C
c_id type2
1 CC

查询结果显示:
1 BB CC
2 null CC
3 BB null
4 null null

[dawugui]

select a.a_id,
       (select b.type1 from b where b.b_id = a.b_id),
       (select c.type1 from c where c.c_id = a.c_id)
from a
       

[dawugui]

create table A(a_id int,b_id int,c_id int)
insert into a values(1 ,1    ,1)
insert into a values(2 ,null ,1)
insert into a values(3 ,1    ,null)
insert into a values(4 ,null ,null)
create table B(b_id int,type1 varchar(10))
insert into b values(1 ,'BB')
create table C(c_id int,type2 varchar(10))
insert into c values( 1 ,'CC')
go

--方法一
select a.a_id,
       type1 = (select b.type1 from b where b.b_id = a.b_id),
       type2 = (select c.type2 from c where c.c_id = a.c_id)
from a

--方法二
select a.a_id,
       b.type1,
       c.type2
from a left join b on a.b_id = b.b_id
left join c on a.c_id = c.c_id
drop table a , b, c

/*
a_id        type1      type2      
----------- ---------- ---------- 
1           BB         CC
2           NULL       CC
3           BB         NULL
4           NULL       NULL

（所影响的行数为 4 行）

*/

[sweetbbs]

select a_id,type1,type2 from a
left join b on a.b_id=b.b_id left join c on a.c_id=c.c_id

[njlywy]

select a.a_id,b.type1,c.type2
from a left join b on a.b_id = b.b_id
left join c on a.c_id = c.c_id
drop table a , b, c