create table t(c varchar(10))
insert t
select 'a'
union all select 'a'
union all select 'b'
union all select 'b'
union all select 'c'
union all select 'c'
declare @t table(a varchar(10))
insert into @t
select 'a'
union all select 'a'
union all select 'b'
union all select 'b'
union all select 'c'
union all select 'c'
select distinct a from @t
union all
select distinct a from @t
/*
(所影响的行数为 6 行)