这样的SQL语句怎么写？

[louifox]

table1:
id1 name id2
1 xxx
2 yyy
3 zzz 3

table2:
id2 flag
1 a
2 b
3 c
查询结果为：
id1 name flag
1 xxx
2 yyy
3 zzz c

如何写SQL语句？

[123456754321]

select t1.id1,t1.name,case(when exist(select table2.flag from table2 where t2.id2=t1.id2 then select table2.flag from table2 where t2.id2=t1.id2 else '' end )
from table1 t1

[mrh123]

使用"左外部连接"试试:

select table1.id1,table1.name,table2.flag
from table1
left outer join
table2
on tabl1.id2=table2.id2

[hdslah]

select a.id1,a.name,b.flag from table1 a left join table2 b on a.id2=b.id2

[wzh1215]

select a.id1,a.name,isnull(b.flag,'') as flag from table1 a left join table2 b on a.id2=b.id2

[伍子V5]

select a.id1,a.name,isnull(b.flag,'') as flag from table1 a left join table2 b on a.id2=b.id2

[txlicenhe]

Select b.*,c.flag
from table2 b
left join table1 on a.id1 = b.id1
left join table2 c on a.id2 = c.id2

[ysycrazy]

select a.id1,a.name,b.flag from table1 a left outer join table2 b on a.id2=b.id2

[yujohny]

Select A.*,B.flag from table1 A left join table2 B on B.id2 = A.id2

[dafu71]

select id1,name,isnull(table2.flag,'') from table1
left join table2 on table1.id2=table2.id2

[sql1]

select a.idl,a.name,b.flag from table1 a left join table2 b on a.id2=b.id2

[zeden]

Select b.*,c.flag
from table2 b
left join table1 on a.id1 = b.id1
left join table2 c on a.id2 = c.id2