MySql的相关子查询执行

Question

对于下面的查询：

SELECT Id, Salary 
FROM Employee e 
WHERE 2 = (SELECT COUNT(DISTINCT Salary) FROM Employee p 
      WHERE e.Salary<=p.Salary) 

不应该在子查询的e.salary<=p.salary状况得到扭转。我们应该重复比较p.salary列与外部查询中的e.salary列，因此p.salary必须位于左侧。如果我的理解错误，请让我知道。

PS：上述查询试图从Employee表中查找第二高薪水。

编辑：这个问题似乎在我的理解。我在下面提到我如何感觉查询可能被执行：

假设我们有一个雇员，有4个薪水10,20,30,40。现在记录开始逐一处理。所以我们得到以下4个中间实现：

`code` 
SELECT Id, Salary 
FROM Employee e 
WHERE 2 = (SELECT COUNT(DISTINCT p.Salary) 
      FROM Employee p 
      WHERE 10<=p.salary -- record with salary 10 
     ); 
SELECT Id, Salary 
FROM Employee e 
WHERE 2 = (SELECT COUNT(DISTINCT p.Salary) 
      FROM Employee p 
      WHERE 20<=p.salary 
     ); 
SELECT Id, Salary 
FROM Employee e 
WHERE 2 = (SELECT COUNT(DISTINCT p.Salary) 
      FROM Employee p 
      WHERE 30<=p.salary 
     ); 
SELECT Id, Salary 
FROM Employee e 
WHERE 2 = (SELECT COUNT(DISTINCT p.Salary) 
      FROM Employee p 
      WHERE 40<=p.salary 
     ); 
`code` 

这个命令是否正确？不应该我们试图比较的列，即p.salary必须在左侧，如p.salary>=e.salary？

Answer 1

这是查询：

SELECT Id, Salary 
FROM Employee e 
WHERE 2 = (SELECT COUNT(DISTINCT p.Salary) 
      FROM Employee p 
      WHERE e.Salary <= p.Salary 
     ); 

据计数的不同薪水是大于或等于任何给定雇员的工资的数目。也就是说，查询会计算考虑关系的第二大薪水。

如果你“逆转”了条件，你会做其他事情。

SELECT Id, Salary 
FROM (SELECT e.*, 
      DENSE_RANK() OVER (ORDER BY e.salary) as dr 
     FROM Employee e 
    ) e 
WHERE dr = 2;