distinct 去重 或group by 分组
找出所有员工当前薪水salary情况
http://www.nowcoder.com/questionTerminal/ae51e6d057c94f6d891735a48d1c2397
select distinct(salary) from salaries
where to_date='9999-01-01'
order by salary desc;
select distinct(salary) from salaries
where to_date='9999-01-01'
group by salary
order by salary desc;
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