--有TAB1
BH    NAME     ID1      ID2
1     NAME1    1001     20001
2     NAME2    1002     20002
3     NAME3    1003     20003
4     NAME4    1004     20004
5     NAME5    1005     20005
.....

--TAB2
XH    NAME     ID1       ID2
1     NAME1             20001
2     NAME1             20001
2     NAME2             20002
3     NAME3             20003
4     NAME4             20004
5     NAME5             20005
.....

--通过ID2关联，得到TAB2
XH    NAME     ID1       ID2
1     NAME1    1001     20001
2     NAME1    1001     20001
2     NAME2    1002     20002
3     NAME3    1003     20003
4     NAME4    1004     20004
5     NAME5    1005     20005
.....

--> 测试数据：[TAB1]
if object_id('[TAB1]') is not null 
drop table [TAB1]
create table [TAB1](
[BH] int,
[NAME] varchar(5),
[ID1] int,
[ID2] int
)
insert [TAB1]
select 1,'NAME1',1001,20001 union all
select 2,'NAME2',1002,20002 union all
select 3,'NAME3',1003,20003 union all
select 4,'NAME4',1004,20004 union all
select 5,'NAME5',1005,20005
--> 测试数据：[TAB2]
if object_id('[TAB2]') is not null 
drop table [TAB2]
create table [TAB2](
[XH] int,
[NAME] varchar(5),
[ID1] int,
[ID2] int
)
insert [TAB2]
select 1,'NAME1',null,20001 union all
select 2,'NAME1',null,20001 union all
select 2,'NAME2',null,20002 union all
select 3,'NAME3',null,20003 union all
select 4,'NAME4',null,20004 union all
select 5,'NAME5',null,20005

update [TAB2]
set [ID1]=a.[ID1] from [TAB1] a
where a.NAME=[TAB2].NAME and a.ID2=[TAB2].ID2

select * from [TAB2]
/*
XH    NAME    ID1    ID2
1    NAME1    1001    20001
2    NAME1    1001    20001
2    NAME2    1002    20002
3    NAME3    1003    20003
4    NAME4    1004    20004
5    NAME5    1005    20005
*/