如何利用标准高效查询SQL实现单表去重A与B字段的交叉值

Question

原始数据和处理后数据输出如下（用标准SQL语言实现）：

提供可验证正确的SQL。

补充数据

select 'a' as col1,'b' as col2 from dual union all
select 'b' as col1,'a' as col2 from dual union all

select 'b' as col1,'c' as col2 from dual union all

select 'b' as col1,'d' as col2 from dual union all

select 'ba' as col1,'ab' as col2 from dual union all
select 'ab' as col1,'ba' as col2 from dual union all

select 'ba' as col1,'bd' as col2 from dual union all

select '你好' as col1,'我好' as col2 from dual union all
select '我好' as col1,'你好' as col2 from dual union all

select '你好我' as col1,'好' as col2 from dual union all
select '你' as col1,'好我好' as col2 from dual union all

select '你好' as col1,'好我' as col2 from dual

预期结果：

COL1	COL2
a	b
b	c
b	d
ab	ba
ba	bd
你好	好我
你好	我好
你	好我好
你好我	好

Answer 1

是a字段或b字段里面出现相同的只留一个吗？