= null和is null区别

= null

判断某个值是否和空相等

is null和is not null

判断某个值为否为空

举例

select e.name, b.bonus
from employee e
left join bonus b
on e.empId = b.empId
where b.bonus < 1000 or b.bonus is null

select e.name, b.bonus
from employee e
left join bonus b
on e.empId = b.empId
where b.bonus < 1000 or b.bonus = null