数据库--leetcode练习176-185-优快云博客

本文链接：https://blog.youkuaiyun.com/sinat_41944023/article/details/96330938

本文介绍了一系列SQL查询技巧，包括获取第二高及第N高的薪水、分数排名、查找连续数字、筛选超过经理收入的员工、发现重复电子邮箱、识别从未下单的客户、找出部门最高及前三高薪员工等。

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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 |
+---------------------+

SELECT Max(Salary) as SecondHighestSalary FROM Employee
WHERE Salary <> (SELECT Salary from Employee 
                ORDER BY Salary DESC LIMIT 1);

177. 第N高的薪水

编写一个 SQL 查询，获取 Employee 表中第 n 高的薪水（Salary）。

+----+--------+
| Id | Salary |
+----+--------+
| 1 | 100 |
| 2 | 200 |
| 3 | 300 |
+----+--------+
例如上述 Employee 表，n = 2 时，应返回第二高的薪水 200。如果不存在第 n 高的薪水，那么查询应返回 null。

+------------------------+
| getNthHighestSalary(2) |
+------------------------+
| 200 |
+------------------------+

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
  RETURN (
      # Write your MySQL query statement below.
  SELECT IFNULL(
  (SELECT Salary FROM Employee e1
  WHERE 
  (SELECT COUNT(DISTINCT Salary) FROM Employee e2 WHERE e2.Salary > e1.Salary)=N-1 LIMIT 1), NULL) AS  NthHighestSalary
      );
END

178. 分数排名

编写一个 SQL 查询来实现分数排名。如果两个分数相同，则两个分数排名（Rank）相同。请注意，平分后的下一个名次应该是下一个连续的整数值。换句话说，名次之间不应该有“间隔”。

+----+-------+
| Id | Score |
+----+-------+
| 1 | 3.50 |
| 2 | 3.65 |
| 3 | 4.00 |
| 4 | 3.85 |
| 5 | 4.00 |
| 6 | 3.65 |
+----+-------+
例如，根据上述给定的 Scores 表，你的查询应该返回（按分数从高到低排列）：

+-------+------+
| Score | Rank |
+-------+------+
| 4.00 | 1 |
| 4.00 | 1 |
| 3.85 | 2 |
| 3.65 | 3 |
| 3.65 | 3 |
| 3.50 | 4 |
+-------+------+

# Write your MySQL query statement below
SELECT s1.Score, COUNT(DISTINCT s2.Score) AS Rank
FROM Scores s1 JOIN Scores s2 
ON s1.Score <= s2.Score
GROUP BY s1.ID
ORDER BY Rank

180. 连续出现的数字

编写一个 SQL 查询，查找所有至少连续出现三次的数字。

+----+-----+
| Id | Num |
+----+-----+
| 1 | 1 |
| 2 | 1 |
| 3 | 1 |
| 4 | 2 |
| 5 | 1 |
| 6 | 2 |
| 7 | 2 |
+----+-----+
例如，给定上面的 Logs 表， 1 是唯一连续出现至少三次的数字。

+-----------------+
| ConsecutiveNums |
+-----------------+
| 1 |
+-----------------+

# Write your MySQL query statement below
SELECT DISTINCT l1.num as ConsecutiveNums
FROM Logs l1, Logs l2, Logs l3 
WHERE l1.ID=l2.ID+1=l3.ID+2 AND l1.num=l2.num=l3.num;

181. 超过经理收入的员工

Employee 表包含所有员工，他们的经理也属于员工。每个员工都有一个 Id，此外还有一列对应员工的经理的 Id。

+----+-------+--------+-----------+
| Id | Name | Salary | ManagerId |
+----+-------+--------+-----------+
| 1 | Joe | 70000 | 3 |
| 2 | Henry | 80000 | 4 |
| 3 | Sam | 60000 | NULL |
| 4 | Max | 90000 | NULL |
+----+-------+--------+-----------+
给定 Employee 表，编写一个 SQL 查询，该查询可以获取收入超过他们经理的员工的姓名。在上面的表格中，Joe 是唯一一个收入超过他的经理的员工。

+----------+
| Employee |
+----------+
| Joe |
+----------+

# Write your MySQL query statement below
SELECT e1.Name AS Employee FROM
Employee e1 JOIN Employee e2 ON e1.Id = e2.Id
WHERE e1.ManagerId IS NOT NULL AND 
e1.Salary > (
    SELECT e2.Salary FROM Employee e2 WHERE e2.Id = e1.ManagerId);

182. 查找重复的电子邮箱（简单）

编写一个 SQL 查询，查找 Person 表中所有重复的电子邮箱。

示例：

+----+---------+
| Id | Email |
+----+---------+
| 1 | a@b.com |
| 2 | c@d.com |
| 3 | a@b.com |
+----+---------+
根据以上输入，你的查询应返回以下结果：

+---------+
| Email |
+---------+
| a@b.com |
+---------+

# Write your MySQL query statement below
SELECT Email FROM Person 
GROUP BY Email
HAVING COUNT(Email)>1;

183. 从不订购的客户（简单）

某网站包含两个表，Customers 表和 Orders 表。编写一个 SQL 查询，找出所有从不订购任何东西的客户。

Customers 表：

+----+-------+
| Id | Name |
+----+-------+
| 1 | Joe |
| 2 | Henry |
| 3 | Sam |
| 4 | Max |
+----+-------+
Orders 表：

+----+------------+
| Id | CustomerId |
+----+------------+
| 1 | 3 |
| 2 | 1 |
+----+------------+

# Write your MySQL query statement below
SELECT DISTINCT a.Name AS Customers FROM 
Customers a LEFT JOIN Orders b ON a.Id=b.CustomerId
WHERE b.CustomerId IS NULL;

184. 部门工资最高的员工（中等）

Employee 表包含所有员工信息，每个员工有其对应的 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 表包含公司所有部门的信息。

+----+----------+
| Id | Name |
+----+----------+
| 1 | IT |
| 2 | Sales |
+----+----------+
编写一个 SQL 查询，找出每个部门工资最高的员工。例如，根据上述给定的表格，Max 在 IT 部门有最高工资，Henry 在 Sales 部门有最高工资。

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

# Write your MySQL query statement below
SELECT b.Name AS Department, a1.Name AS Employee, a1.Salary AS Salary
FROM Employee a1 JOIN Department b ON a1.DepartmentId = b.Id
WHERE a1.Salary IN (SELECT MAX(Salary) FROM Employee a2 WHERE a1.DepartmentId=a2.DepartmentId);

185. 部门工资前三高的员工（困难）

Employee 表包含所有员工信息，每个员工有其对应的 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 表包含公司所有部门的信息。

+----+----------+
| Id | Name |
+----+----------+
| 1 | IT |
| 2 | Sales |
+----+----------+
编写一个 SQL 查询，找出每个部门工资前三高的员工。例如，根据上述给定的表格，查询结果应返回：

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

185. 部门工资前三高的员工（困难）

Employee 表包含所有员工信息，每个员工有其对应的 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 表包含公司所有部门的信息。

# Write your MySQL query statement below
SELECT b.Name as Department,
       a1.Name as Employee,
       a1.Salary AS Salary
FROM  Employee a1 JOIN Department b ON a1.DepartmentId=b.Id
WHERE  ( SELECT COUNT(DISTINCT a2.Salary) FROM Employee a2 WHERE a2.Salary>a1.Salary AND a1.DepartmentId=a2.DepartmentId )<3
ORDER BY Department, Salary DESC ;

思路：自我合拼（自己和自己JOIN），比较同部门中比自己收入高的人，若不超过3个，那自己，就是在前三。