leetcode SQL题目

Easy

175.组合两个表
左连接

select Person.FirstName, Person.LastName, Address.City, Address.State
from Person
left join Address
on Person.PersonId = Address.PersonId

176.第二高的薪水
先选出最大的，再选出第二大的

SELECT MAX(Salary) SecondHighestSalary
FROM Employee
WHERE Salary<(SELECT MAX(Salary) FROM Employee) //where条件里是薪水小于最大薪水

181.超过经理收入的员工
表自身按不同属性进行内连接
当一个表要比较自身的不同列的时候可以对自身进行表连接

SELECT e.Name as Employee
FROM Employee as e
JOIN Employee as m
ON e.ManagerId = m.Id
WHERE e.Salary>m.Salary

182.查找重复的电子邮箱
在 SQL 中增加 HAVING 子句原因是，WHERE 关键字无法与聚合函数一起使用。
HAVING 子句可以让我们筛选分组后的各组数据。

SELECT Email
FROM Person
GROUP BY Email
HAVING COUNT(*)>1

183.从不订购的客户

SELECT Name as Customers from Customers
LEFT JOIN Orders
ON Customers.Id = Orders.CustomerId
WHERE Orders.CustomerId IS NULL;

196.删除重复的电子邮件
参考：
https://leetcode.com/problems/delete-duplicate-emails/discuss/55614/A-skillful-mysql-solution-avoid-"-select-and-update-conflict"

https://leetcode.com/problems/delete-duplicate-emails/discuss/55553/Simple-Solution

197.上升的温度
表自身连接

SELECT t1.Id
FROM Weather t1
INNER JOIN Weather t2
ON TO_DAYS(t1.Date) = TO_DAYS(t2.Date) + 1
WHERE t1.Temperature > t2.Temperatur

595.大的国家
参考：
https://leetcode.com/problems/big-countries/discuss/103561/Union-and-OR-and-the-Explanation

用Union代替Or，加快速度（适用于查询不同列的情况，如果是同一列则OR快）

596.超过5名学生的课
普通的group by,每组统计的时候用distinct去重

627.交换工资
https://leetcode.com/problems/swap-salary/discuss/104716/Short-and-Simple

https://leetcode.com/problems/swap-salary/discuss/104722/Simple-and-short-with-IF

Medium

177.第N高的薪水

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
DECLARE M INT;
SET M=N-1;
  RETURN (
      SELECT DISTINCT Salary
      FROM Employee
      ORDER BY Salary DESC
      LIMIT 1 OFFSET M     
  );
END

LIMIT m OFFSET n 表示从第n个记录开始取m条（第一个记录编号是0）
参考：
https://leetcode.com/problems/nth-highest-salary/discuss/53041/Accpted-Solution-for-the-Nth-Highest-Salary

178.分数排名

SELECT Scores.Score, COUNT(Ranking.Score) AS RANK 
  FROM Scores   //Score表
     , (
       SELECT DISTINCT Score
         FROM Scores
       ) Ranking  //Rankin表，表中存放所有不重复的成绩
       //FROM对两个表进行了笛卡尔积，假设一共有4个不同的成绩，score表中的每一行被复制4次，并添加成绩一列
 WHERE Scores.Score <= Ranking.Score  //用来计算Score中每个成绩分别对应的名次，假设当前行的成绩是2，所有的成绩是1,2，3,4，那么他的名次就是第3名，他小于1,2，3,4，中的三个数
 GROUP BY Scores.Id, Scores.Score
 ORDER BY Scores.Score DESC;

180.连续出现的数字
参考：（好理解）
https://leetcode.com/problems/consecutive-numbers/discuss/53418/Simple-solution

Select DISTINCT l1.Num As ConsecutiveNums
from Logs l1, Logs l2, Logs l3   //三个表连接
where l1.Id=l2.Id-1 and l2.Id=l3.Id-1  //取Id相连的行
and l1.Num=l2.Num and l2.Num=l3.Num  //取数字相同的行

参考：（时间占用最小，至于为什么不太清楚）
https://leetcode.com/problems/consecutive-numbers/discuss/282107/Simple-MySql-Tuple-Check.-Fast

SELECT Distinct(Num) as ConsecutiveNums
FROM Logs
WHERE (ID+1,Num) IN (Select Id, Num From Logs)
AND (ID+2,Num) IN (Select Id, Num From Logs)
//用两次表查询

184.部门工资最高的员工
参考：
https://leetcode.com/problems/department-highest-salary/discuss/53607/Three-accpeted-solutions
这里取的第1,3种方法

SELECT D.Name AS Department ,E.Name AS Employee ,E.Salary 
FROM
	Employee E,
	(SELECT DepartmentId,max(Salary) as max FROM Employee GROUP BY DepartmentId) T,  //将部门以及最高工资提取成一个表
	Department D   //三个表连接
WHERE E.DepartmentId = T.DepartmentId 
  AND E.Salary = T.max
  AND E.DepartmentId = D.id

SELECT D.Name AS Department ,E.Name AS Employee ,E.Salary 
from 
	Employee E,
	Department D 
WHERE E.DepartmentId = D.id 
  AND (DepartmentId,Salary) in 
  (SELECT DepartmentId,max(Salary) as max FROM Employee GROUP BY DepartmentId) //将部门以及最高工资提取成一个表

626.换座位

/* get all the even numbered rows as odd numbered rows */
SELECT s1.id - 1 as id, s1.student
FROM Seat s1
WHERE s1.id MOD 2 = 0   //先凑出偶数id的

UNION

/* get all the odd numbered rows as even numbered rows */
SELECT s2.id + 1 as id, s2.student
FROM Seat s2
WHERE s2.id MOD 2 = 1 AND s2.id != (SELECT MAX(id) FROM Seat)  //凑出奇数id的，但是不处理最后一行，因为每次都是以1开头的，那么如果最后一个以奇数结尾，说明一共有奇数行，则最后一行不需要处理
/* Just don't get the last row as we will handle it in the next UNION */

UNION

/* get the last row if odd and don't change the id value */
SELECT s3.id, s3.student
FROM Seat s3
WHERE s3.id MOD 2 = 1 AND s3.id = (SELECT MAX(id) FROM Seat) //如果是奇数行，直接取最后一行奇数

/* Order the result by id */
ORDER BY id ASC;

参考：
https://leetcode.com/problems/exchange-seats/discuss/104707/Using-two-UNION-operators