-- A='', B='', A 和 B 会转为 null，比较结果 xxx：ccc, A 既不等于 B，也不不等于 B， 处于不可比较的一种状态

with t1 as (select '' a, '' b from dual )

select t1.*, case when t1.a=t1.b then 'aaa' when t1.a!=t1.b then 'bbb' else 'ccc' end xxx from t1;

-- A='', B='1'， A 会转为 null， 比较结果 xxx：ccc, 虽然 B 有值，但仍然处于不可比较的一种状态

with t1 as (select '' a, '1' b from dual )

select t1.*, case when t1.a=t1.b then 'aaa' when t1.a!=t1.b then 'bbb' else 'ccc' end xxx from t1;

-- A='', B=null, A 会转为 null， 比较结果 xxx：ccc, 仍然处于不可比较的一种状态

with t1 as (select '' a, null b from dual )

select t1.*, case when t1.a=t1.b then 'aaa' when t1.a!=t1.b then 'bbb' else 'ccc' end xxx from t1;

-- A=null， B=null， 比较结果 xxx：ccc, 仍然处于不可比较的一种状态

with t1 as (select null a, null b from dual )

select t1.*, case when t1.a=t1.b then 'aaa' when t1.a!=t1.b then 'bbb' else 'ccc' end xxx from t1;

-- 结论： oracle 的 null 不可比较， 或许可以转换一下使用

with t1 as (select '' a, '' b from dual )

select t1.*, case when nvl(t1.a, '9999')=nvl(t1.b, '9999') then 'aaa'

                  when nvl(t1.a, '9999')!=nvl(t1.b, '9999') then 'bbb'

                  else 'ccc'

    end xxx from t1;

with t1 as (select '' a, '' b from dual )

select t1.*, case when decode(t1.a, '', '2')=decode(t1.b, '', '2') then 'aaa'

                  when decode(t1.a, '', '2')!=decode(t1.b, '', '2') then 'bbb' else 'ccc' end xxx from t1;