declare @A table (c1 int,c2 varchar(1))
insert into @A
select 1,'a' union all
select 2,'b' union all
select 3,'c' union all
select 4,'d'

declare @B table (c1 int,c2 varchar(1))
insert into @B
select 1,'a' union all
select 2,'b' union all
select 3,'c' union all
select 4,'d' union all
select 5,'e' union all
select 6,'f' union all
select 7,'g' union all
select 8,'h' union all
select 9,'i' union all
select 10,'j'

declare @i int set @i=0
update @B set c2=(select top 1 c1 from @A order by newid()),@i=@i+1
select * from @B
/*
c1          c2
----------- ----
1           4
2           2
3           3
4           1
5           4
6           2
7           4
8           2
9           1
10          2
*/