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deletefrom Personwhere Id in ( select Id from ( select Id, row_number() over(partition by Email order by Id) rn from Person ) t1 where rn>1 )使用窗口函数解题窗口函数可以解决什么问题？排名：每个班级按成绩来排名top N：找出每个学科排名前N的同学进行奖励总之：在每组内排名的问题.

select c.`Name` as Customersfrom Orders as oright join Customers as con c.`Id` = o.`CustomerId`where o.`CustomerId` is null

select p.`Email`from Person as pgroup by p.`Email`having count(*)>1对Email出现的次数进行计数，大于1的就说明重复了count(*)可以不出现在选择列表里

select a.`Name` as Employeefrom Employee as a,Employee as bwhere a.`ManagerId` = b.`id`and a.`Salary` > b.`Salary` 使用自连接解题，根据筛选条件得到有经理的员工的经理的薪资，再比较员工和其经理的薪资。...

select ifnull( (select distinct salary as SecondHighestSalary from Employee order by salary desc limit 1 offset 1) ,null)as SecondHighestSalary要点第二高的薪水可能为空，用ifnull解决。内层select语句形成了一个临时表.

两种代码执行性能有不小的差异第一种select p.`FirstName` ,p.`LastName` ,a.`City` ,a.`State`from Person as pleft join Address as aon p.`PersonId` = a.`PersonId`在这种方法中，查询列表规定了所查询字段的表名，性能如下：第二种select FirstName ,LastName ,City