【sql】第二高的工资 Second Highest Salary

问题：

Write a SQL query to get the second highest salary from the Employee table.

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

For example, given the above Employee table, the query should return 200 as the second highest salary. If there is no second highest salary, then the query should return null.

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

解决：

① 先来看一种使用Limit和Offset两个关键字的解法，MySQL中Limit后面的数字限制了咱们返回数据的个数，Offset是偏移量，那么若是咱们想找第二高薪水，咱们首先能够先对薪水进行降序排列，而后咱们将Offset设为1，那么就是从第二个开始，也就是第二高薪水，而后咱们将Limit设为1，就是只取出第二高薪水，若是将Limit设为2，那么就将第二高和第三高薪水都取出来。1060 ms

SELECT Salary AS SecondHighestSalary FROM Employee
GROUP BY SecondHighestSalary
UNION ALL(SELECT NULL AS SecondHighestSalary)
ORDER BY SecondHighestSalary DESC
LIMIT 1
OFFSET 1;
/*使用UNION ALL保证若是第二大的工资不存在，就返回null
    LIMIT表示取返回结果的一个值
    OFFSET表示从何处开始取
    ORDER BY不能用Salary做为条件，由于此时使用SecondHighestSalary标识*/

【注】AS 子句可用来更改结果集列的名称或为派生列分配名称。使用GROUP BY保证工资值不存在重复，因此也能够使用DISTINCT排除重复的值。

SELECT DISTINCT Salary  SecondHighestSalary
FROM Employee
UNION ALL (SELECT NULL AS SecondHighestSalary)
ORDER BY SecondHighestSalary DESC
LIMIT 1
OFFSET 1;

② 能够使用Max函数返回最大值，逻辑是咱们取出的不包含最大值的数字中的最大值，即为第二大值。 1384 ms

SELECT MAX(Salary) As SecondHighestSalary FROM Employee
WHERE Salary NOT IN
(SELECT MAX(Salary) FROM Employee);

③ 就是用小于号<代替了Not in关键字，效果相同。914 ms

SELECT MAX(Salary) As SecondHighestSalary FROM Employee
WHERE Salary <
(SELECT MAX(Salary) FROM Employee);

④ 能够扩展到找到第N高的薪水的方法，只要将下面语句中的1改成N-1便可，第二高的薪水带入N-1就是1，下面语句的逻辑是，假如咱们要找第二高的薪水，那么咱们容许其中一个最大值存在，而后在其他的数字中找出最大的，即为整个的第二大的值；1421 ms

SELECT MAX(Salary) As SecondHighestSalary FROM Employee E1
WHERE 1 = 
(SELECT COUNT(DISTINCT(E2.Salary)) FROM Employee E2 
    WHERE E2.Salary > E1.Salary);