update 表A
set 列b=(select top 1 m列 from 表B where a列=t.a列),
    列c=(select top 1 c列 from 表B where a列=t.a列)
from 表A t

------解决方案--------------------
SQL code

--> 测试数据: @A
declare @A table (a int,b varchar(1),c varchar(1))
insert into @A
select 1,null,null union all
select 2,null,null union all
select 3,null,null union all
select 4,null,null

--> 测试数据: @B
declare @B table (a int,m varchar(1),n varchar(1))
insert into @B
select 1,'a','j' union all
select 1,'b','k' union all
select 2,'c','l' union all
select 2,'d','m' union all
select 3,'e','n' union all
select 3,'f','o' union all
select 3,'g','p' union all
select 4,'h','q' union all
select 4,'i','r'

--更新前
select * from @A
/*
a           b    c
----------- ---- ----
1           NULL NULL
2           NULL NULL
3           NULL NULL
4           NULL NULL
*/

--更新后
update @A
set b=(select top 1 m from @B where a=t.a),
    c=(select top 1 n from @B where a=t.a)
from @A t

select * from @A
/*
a           b    c
----------- ---- ----
1           a    j
2           c    l
3           e    n
4           h    q
*/