select decode(null,null,'等','不等') from dual;
select case when null = null then '等' else '不等' end from dual;

这上面两句，执行的结果不同.

select decode(null,null,'等','不等') from dual;
select case when null is null then '等' else '不等' end from dual;

这上面两句，执行的结果相同.

select sysdate from dual where null is null;--true
select sysdate from dual where '' is null;--true

select sysdate from dual where null = null;--false
select sysdate from dual where '' = '';--false