MySql数据库ifnull的用法

select
    ifnull(
    (select distinct Salary as SecondHighestSalary
    from Employee 
    order by Salary desc
    limit 1 offset 1),null) as SecondHighestSalary;

ifnull函数的参数有两个，如果不是null，返回第一个参数，如果为null返回第二个参数