简单的SQL语句问题？

[sujiang]

有表A、B、C
表A字段
ID zd1
1 56
2 43
3 65
4 23
5 67
表B字段
AID zd2
3 78
4 33
表C字段
AID zd3
2 90
3 87
5 67
求一SQL语句,条件：A.ID<=4; B.AID为A.ID; C.AID为A.ID 并且表B和C中没有AID的字段显示为空。
根据条件结果要为：
ID zd1 zd2 zd3
1 56
2 43 90
3 65 78 87
4 23 33

不用外关联（JOIN）只用内关联。

[tim_spac]

楼主不必担心，SQL Server会帮你做一些优化。只要相关的ID做好索引(最好是PK/Unique)，应该不会超时

[$扫地僧$]

select A.ID,
       A.Zd1,
       (select Zd2 from B where A.iD=Aid) as Zd2,
       (select Zd3 from c where A.iD=Aid) as Zd3
 from A 
where ID<=4

[dawugui]

create table A(ID int,             zd1 int)
insert into A values(1,               56) 
insert into A values(2,               43) 
insert into A values(3,               65) 
insert into A values(4,               23) 
insert into A values(5,               67) 
create table B(AID int,             zd2 int)
insert into B values(3,               78) 
insert into B values(4,               33) 
create table C(AID int,             zd3 int)
insert into C values(2,               90 )
insert into C values(3,               87 )
insert into C values(5,               67 )
go

select A.* , B.zd2 , C.zd3 from A left join B on A.id = B.aid left join C on A.id = C.aid where a.id <= 4

drop table A,B,C

/*
ID          zd1         zd2         zd3
----------- ----------- ----------- -----------
1           56          NULL        NULL
2           43          NULL        90
3           65          78          87
4           23          33          NULL

(4 行受影响)
*/

[-狙击手-]

 
select ID,zd1,
zd2 = isnull(select zd2 from tableb where a.id = aid),''),
zd3 = isnull(select zd3 from tablec where a.id = aid),'')
from tablea a

[-狙击手-]

select ID,zd1,
zd2 = isnull(select zd2 from tableb where a.id = aid),''),
zd3 = isnull(select zd3 from tablec where a.id = aid),'')
from tablea a
where id <=4

[dawugui]

create table A(ID int,             zd1 int)
insert into A values(1,               56) 
insert into A values(2,               43) 
insert into A values(3,               65) 
insert into A values(4,               23) 
insert into A values(5,               67) 
create table B(AID int,             zd2 int)
insert into B values(3,               78) 
insert into B values(4,               33) 
create table C(AID int,             zd3 int)
insert into C values(2,               90 )
insert into C values(3,               87 )
insert into C values(5,               67 )
go

select A.* , isnull(cast(B.zd2 as varchar),'') zd2 , isnull(cast(C.zd3 as varchar),'') zd3 from A left join B on A.id = B.aid left join C on A.id = C.aid where a.id <= 4

drop table A,B,C

/*
ID          zd1         zd2                            zd3
----------- ----------- ------------------------------ ------------------------------
1           56                                         
2           43                                         90
3           65          78                             87
4           23          33                             

(4 行受影响)

*/

[dobear_0922]

select ID, zd1, zd2, zd3  
from A left join B on A.ID=B.AID
	left join C on A.ID=C.AID
where ID<=4

[areswang]

DECLARE @A TABLE(ID INT,ZD1 INT)
INSERT INTO @A SELECT 1,56
UNION ALL
SELECT 2,43
UNION ALL
SELECT 3,65
UNION ALL
SELECT 4,23
UNION ALL
SELECT 5,67

DECLARE @B TABLE(AID INT,zd2 INT)
INSERT INTO @B SELECT 3,78
UNION ALL
SELECT 4,33

DECLARE @C TABLE(AID INT,zd3 INT)
INSERT INTO @C SELECT 2,90
UNION ALL
SELECT 3,87
UNION ALL
SELECT 5,67

select ID,zd1,
zd2 = isnull((select zd2 from @b where a.id = aid),''),
zd3 = isnull((select zd3 from @c where a.id = aid),'')
from @a a
where id <=4

[dobear_0922]

create table A(ID int,             zd1 int)
insert into A values(1,               56) 
insert into A values(2,               43) 
insert into A values(3,               65) 
insert into A values(4,               23) 
insert into A values(5,               67) 
create table B(AID int,             zd2 int)
insert into B values(3,               78) 
insert into B values(4,               33) 
create table C(AID int,             zd3 int)
insert into C values(2,               90 )
insert into C values(3,               87 )
insert into C values(5,               67 )
go

select ID, zd1=isnull(rtrim(zd1), ''), zd2=isnull(rtrim(zd2), ''), zd3=isnull(rtrim(zd3), '')  
from A left join B on A.ID=B.AID
	left join C on A.ID=C.AID
where ID<=4

drop table a,b,c

/*
ID          zd1          zd2          zd3
----------- ------------ ------------ ------------
1           56                        
2           43                        90
3           65           78           87
4           23           33           

(4 row(s) affected)
*/

[sujiang]

用外部关联left join和不用外部关联。
哪个快点呢。我的数据表很多有A、B、C、D、E、F、G
有的表有几百条记录，有的几千条，字段也多。用外部关联LEFT JOIN会超时呀。怎么办？？？？