WITH t AS (
SELECT 'a' id FROM dual
UNION ALL 
SELECT 'b' FROM dual
UNION ALL 
SELECT 'c' FROM dual
UNION ALL 
SELECT 'd' FROM dual
)
SELECT t1.id||t2.id FROM t t1,t t2 WHERE t1.id<t2.id;

------解决方案--------------------
自连接做个迪卡尔集，再比较ASCII码大小。

with t as (
--NO NAME
select 1 as num, 'A' as name from dual
union all
select 2, 'B' from dual
union all
select 3, 'C' from dual
union all
select 4, 'D' from dual
)
select aname || bname from (
select a.num as anum,a.name as aname,b.num as bnum, b.name as bname from t a, t b)
where ascii(aname) < ascii(bname)

ANAME||BNAME  
------------  
AB          
AC          
AD          
BC          
BD          
CD