题解 | #平均工资#

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

此题运用了union来进行两个结果集的拼接；得到一个列子查询。