MySql的相關子查詢執行

Question

對於下面的查詢：

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

不應該在子查詢的e.salary<=p.salary狀況得到扭轉。我們應該重複比較p.salary列與外部查詢中的e.salary列，因此p.salary必須位於左側。如果我的理解錯誤，請讓我知道。

PS：上述查詢試圖從Employee表中查找第二高薪水。

編輯：這個問題似乎在我的理解。我在下面提到我如何感覺查詢可能被執行：

假設我們有一個僱員，有4個薪水10,20,30,40。現在記錄開始逐一處理。所以我們得到以下4箇中間實現：

`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` 

這個命令是否正確？不應該我們試圖比較的列，即p.salary必須在左側，如p.salary>=e.salary？

Answer 1

這是查詢：

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

據計數的不同薪水是大於或等於任何給定僱員的工資的數目。也就是說，查詢會計算考慮關係的第二大薪水。

如果你「逆轉」了條件，你會做其他事情。

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