-- in后面只能有一个字段。 -- <>后面只能有一个字段.不能同时与一个查询出两个字段的临时表比较，salary是一个字段，不能和两个字段比较 -- 注意排除在职(to_date = '9999-01-01' )员工

select avg(salary) as avg_salary from salaries where to_date = '9999-01-01' and salary not in (select min(salary) from salaries where to_date = '9999-01-01') and salary not in(select max(salary) from salaries where to_date = '9999-01-01' );