备注-赋值返回null

输出null，所以外面再加一层

select (select distinct salary from Employee order by salary desc limit 1,1) as SecondHighestSalary

我的理解是：当Employee表里只有一条数据时，内层SQL语句查询不到数据，其返回结果是空，而外层SQL的作用是把内层的查询结果赋值给SecondHighestSalary.