--这是小梁写的,其他方法参照你我sql server版的贴.
CREATE TABLE  t(id int,num1 varchar(1),num2 varchar(1))
insert into t select 1,'a','b' UNION ALL
select 2,'b','d' UNION ALL
select 3,'c','e' UNION ALL
select 4,'d','b' UNION ALL
select 5,'a','f'


DECLARE @resualt VARCHAR(200)
SET @resualt='('
SELECT @resualt=@resualt+','+num1 FROM
(
SELECT distinct num1 FROM  t
UNION
SELECT distinct num2 FROM t
) Tt
SELECT REPLACE(@resualt,'(,','(')+')'


drop table t

/*
（所影响的行数为 5 行）

                                                                                                                                                                                                                                                                 
---------------------------------------------------------------------------------------------------------------- 
(a,b,c,d,e,f)

（所影响的行数为 1 行）
*/