mysql查詢之 第二高薪水,部門工資最高的員工，部門工資前三高的員工

（1）第二高薪水

 編寫一個 SQL 查詢，獲取 Employee 表中第二高的薪水（Salary） 

+----+--------+
| Id | Salary |
+----+--------+
| 1  | 100    |
| 2  | 200    |
| 3  | 300    |
+----+--------+

例如上述 Employee 表，SQL查詢應該返回 200 作為第二高的薪水。如果不存在第二高的薪水，那么查詢應返回 null。

+---------------------+
| SecondHighestSalary |
+---------------------+
| 200                 |
+---------------------+

剛一看題目，覺得很簡單的，可是做了一下之后才發現很多細節都沒考慮，特此記錄下來

select (select distinct Salary  from Employee order by Salary desc limit 1,1)as SecondHighestSalary

或

select (select distinct Salary  from Employee order by Salary desc limit 1 offset 1)as SecondHighestSalary

limit限制結果范圍

SELECT * FROM Employee LIMIT 0,1; 
SELECT * FROM Employee LIMIT 1 OFFSET 0;
SELECT * FROM products LIMIT 1;

“0”: 代表數據獲取的起始位置.(0代表第一條記錄,以此遞增) 
“1”: 期望獲取的記錄條數. 

當獲取的記錄是從第一條開始則可以省略書寫起始位置 “0”

（2）部門工資最高的員工

Employee 表包含所有員工信息，每個員工有其對應的 Id, salary 和 department Id。

+----+-------+--------+--------------+
| Id | Name  | Salary | DepartmentId |
+----+-------+--------+--------------+
| 1  | Joe   | 70000  | 1            |
| 2  | Henry | 80000  | 2            |
| 3  | Sam   | 60000  | 2            |
| 4  | Max   | 90000  | 1            |
+----+-------+--------+--------------+

Department 表包含公司所有部門的信息。

+----+----------+
| Id | Name     |
+----+----------+
| 1  | IT       |
| 2  | Sales    |
+----+----------+

編寫一個 SQL 查詢，找出每個部門工資最高的員工。例如，根據上述給定的表格，Max 在 IT 部門有最高工資，Henry 在 Sales 部門有最高工資。

+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT         | Max      | 90000  |
| Sales      | Henry    | 80000  |
+------------+----------+--------+

SELECT d. NAME AS Department,e. NAME AS Employee,e.Salary FROM Department d,Employee e
    WHERE d.Id = e.DepartmentId AND (e.Salary, e.DepartmentId) IN (SELECT Max(Salary),DepartmentId FROM Employee GROUP BY DepartmentId)

（3）部門工資前三高的員工

Employee 表包含所有員工信息，每個員工有其對應的 Id, salary 和 department Id 。

+----+-------+--------+--------------+
| Id | Name  | Salary | DepartmentId |
+----+-------+--------+--------------+
| 1  | Joe   | 70000  | 1            |
| 2  | Henry | 80000  | 2            |
| 3  | Sam   | 60000  | 2            |
| 4  | Max   | 90000  | 1            |
| 5  | Janet | 69000  | 1            |
| 6  | Randy | 85000  | 1            |
+----+-------+--------+--------------+

Department 表包含公司所有部門的信息。

+----+----------+
| Id | Name     |
+----+----------+
| 1  | IT       |
| 2  | Sales    |
+----+----------+

編寫一個 SQL 查詢，找出每個部門工資前三高的員工。例如，根據上述給定的表格，查詢結果應返回：

+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT         | Max      | 90000  |
| IT         | Randy    | 85000  |
| IT         | Joe      | 70000  |
| Sales      | Henry    | 80000  |
| Sales      | Sam      | 60000  |
+------------+----------+--------+

select d.Name as Department,e.Name Employee,e.Salary from Employee e,Department d 
where 
    e.DepartmentId=d.Id and
    Salary>=IFNULL((select distinct e1.Salary from Employee as e1 where e.DepartmentId=e1.DepartmentId order by e1.Salary desc limit 2,1),0) 
order by DepartmentId,Salary desc;

說明：

　　IFNULL(expression_1,expression_2);

　　如果expression_1不為NULL，則IFNULL函數返回expression_1; 否則返回expression_2的結果