力扣数据库题目176第二高的薪水

题目

编写一个 SQL 查询，获取 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查询应该返回 200 作为第二高的薪水。如果不存在第二高的薪水，那么查询应返回 null。

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

来源：力扣（LeetCode）

方案一

SELECT salary SecondHighestSalary
FROM (		
	SELECT DISTINCT salary FROM test.employee 
    UNION ALL 
    SELECT NULL  
    UNION ALL 
	SELECT NULL         
) t ORDER BY salary DESC LIMIT 1,1

方案二

SELECT salary SecondHighestSalary 
FROM (		
	SELECT salary,DENSE_RANK() OVER(ORDER BY salary DESC) num
    FROM test.employee 
    UNION ALL 
    SELECT NULL,2
    UNION ALL 
	SELECT NULL,2    
) t
WHERE num = 2
LIMIT 1;

方案三

SELECT t.salary SecondHighestSalary
FROM (
	SELECT salary  
	FROM test.employee 	
    WHERE salary < (
		SELECT MAX(salary)
        FROM test.employee 
	) 
    UNION ALL 
    SELECT NULL   
) t
ORDER BY t.salary DESC LIMIT 1

方案四

SELECT t.salary SecondHighestSalary
FROM (
	SELECT salary  
	FROM test.employee 	
    WHERE salary < ANY(
		SELECT salary
        FROM test.employee 
	) 
    UNION ALL 
    SELECT NULL   
) t
ORDER BY t.salary DESC LIMIT 1

方案五

SELECT (SELECT salary 
FROM (		
	SELECT DISTINCT salary FROM test.employee        
) t ORDER BY salary DESC LIMIT 1,1)SecondHighestSalary

方案六

SELECT (SELECT salary 
FROM (   
  SELECT salary,DENSE_RANK() OVER(ORDER BY salary DESC) num
  FROM test.employee 
) t
WHERE num = 2
LIMIT 1) SecondHighestSalary

方案七

SELECT (SELECT t.salary 
FROM (
	SELECT salary  
	FROM test.employee 	
    WHERE salary < (
		SELECT MAX(salary)
        FROM test.employee 
	)  
) t
ORDER BY t.salary DESC LIMIT 1) SecondHighestSalary

方案八

SELECT (SELECT t.salary 
FROM (
	SELECT salary  
	FROM test.employee 	
    WHERE salary < ANY(
		SELECT salary
        FROM test.employee 
	) 
) t
ORDER BY t.salary DESC LIMIT 1) SecondHighestSalary

分析

题目要求的结果是一行一列，并保证两个条件

1、列别名SecondHighestSalary

2、行至少一行NULL,为了维持这个问题

前四个方案是制造出来一个NULL行，后四个方案是0行使用select （）子查询后造出NULL行

为了保证条件2，废了很多事。

方案一五使用distinct，二六使用的开窗，三七使用的子查询，四八使用的ANY

建表

CREATE TABLE `employee` (
  `id` int NOT NULL AUTO_INCREMENT,
  `salary` int NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB  COLLATE=utf8mb4_0900_ai_ci;

数据初始化

INSERT INTO `test`.employee(`id`,`salary`)
VALUES(1,100),(2,200),(3,300)