create table taba
(a_1 int, a_2 int)

insert into taba
select 3, 2 union all
select 4, 6 union all
select 7, 8

create function fnyf
(@a1 int,@a2 int)
returns varchar(15)
as
begin
 declare @m int,@i int,@b1 int,@b2 int
 select @m=case when @a1>=@a2 then @a2 else @a1 end,@i=2,@b1=@a1,@b2=@a2
 while(@i<=@m)
 begin
   if @b1%@i=0 and @b2%@i=0
     select @b1=@b1/@i,@b2=@b2/@i
   select @i=@i+1
 end
 return cast(@b1 as varchar)+':'+cast(@b2 as varchar)
end


select dbo.fnyf(a_1,a_2) 'a_1：a_2' from taba

a_1：a_2
---------------
3:2
2:3
7:8

(3 row(s) affected)

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

--> 测试数据: @表A
declare @表A table (a_1 int,a_2 int)
insert into @表A
select 3,2 union all
select 12,18 union all --改成12和18 测试约分
select 7,8

declare @i int,@j int
select @i=max(a_1),@j=max(a_2) from @表A

;with maco as (
select number from master..spt_values where type='p' 
and (number between 2 and @i or number between 2 and @j))
select isnull(b.c1,a.a_1) as a_1,isnull(b.c2,a.a_2) as a_2 from @表A a
left join (
select min(a_1/number) as c1,min(a_2/number) as c2,a_1,a_2 
from maco a,@表A b where a_1%number=0 and a_2%number=0 group by a_1,a_2)b
on a.a_1=b.a_1 and a.a_2=b.a_2

/*
a_1         a_2
----------- -----------
3           2
2           3
7           8
*/