LeetCode数据库部分题目

组合两个表（简单）

解题

SELECT p.FirstName,p.LastName,a.City,a.State
FROM Person p
LEFT JOIN Address a 
ON p.PersonId=a.PersonId;

第二高的薪水（简单）

解题

SELECT 
    IFNULL(
        (SELECT DISTINCT 
            Salary 
        FROM 
            Employee 
        ORDER BY Salary DESC 
        LIMIT 1,1),
        NULL) SecondHighestSalary;

超过经理收入的员工（简单）

解题

SELECT 
    IFNULL(e3.n1,NULL) Employee
FROM
    (SELECT 
        e1.Name n1,e1.Salary s1,e2.Name n2,e2.Salary s2
    FROM
        Employee e1,Employee e2
    WHERE
        e2.Id=e1.ManagerId) e3
WHERE
    e3.s1>e3.s2;

查找重复的电子邮箱（简单）

解题

# Write your MySQL query statement below
SELECT IFNULL(Email,NULL) Email
FROM (SELECT COUNT(Id) count_id,Email FROM Person GROUP BY Email) p1
WHERE p1.count_id>1;

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

从不订购的客户（简单）

解题

# Write your MySQL query statement below
SELECT IFNULL(c.name,NULL) Customers
FROM Customers c
WHERE c.id NOT IN (SELECT o.CustomerId FROM Orders o);

删除重复的电子邮箱（简单）

解题
该做法要注意删除语句不能作用于查询后修改的表，也就是group by之后应该建临时表来保存

DELETE FROM Person
WHERE Id NOT IN (  
    SELECT id FROM
   (
       SELECT MIN(Id) AS Id 
       FROM Person
       GROUP BY Email
   ) AS temp    
);

自连接做法

# Write your MySQL query statement below
DELETE p1
FROM Person p1,Person p2
WHERE p1.Email=p2.Email AND p1.Id>p2.Id;

上升的温度（简单）

日期的比较要用DATEDIFF函数

# Write your MySQL query statement below
SELECT IFNULL(w1.Id,null) Id
FROM Weather w1,Weather w2
WHERE DATEDIFF(w1.RecordDate, w2.RecordDate) = 1 AND w1.Temperature>w2.Temperature

大的国家（简单）

使用OR

# Write your MySQL query statement below
SELECT name,population,area
FROM World
WHERE area>3000000 OR population>25000000

使用UNION连接子查询，UNION默认DISTINCT

SELECT name, population, area
FROM world
WHERE area > 3000000
UNION
SELECT name, population, area
FROM world
WHERE population > 25000000
;

通过对比，发现UNION的速度更快

超过5名学生的课（简单）

# Write your MySQL query statement below
SELECT IFNULL(class,null) class
FROM courses
GROUP BY class
HAVING COUNT(DISTINCT student)>=5

有趣的电影（简单）

• 用%运算，效率低

# Write your MySQL query statement below
SELECT *
FROM cinema
WHERE description<>"boring" AND id%2=1
ORDER BY rating DESC

• 用MOD函数，效率高

# Write your MySQL query statement below
SELECT *
FROM cinema
WHERE description<>"boring" AND MOD(id,2)=1
ORDER BY rating DESC

交换工资（简单）

这个好理解，但是效率低

# Write your MySQL query statement below
UPDATE salary
SET sex=IF(sex='f','m','f');

用CASE表达式

# Write your MySQL query statement below
UPDATE salary
SET sex=CASE sex WHEN 'f' THEN 'm' ELSE 'f' END;

重新格式化部门表（简单）

# Write your MySQL query statement below
SELECT id,
SUM(CASE `month` WHEN 'Jan' THEN revenue END) Jan_Revenue,
SUM(CASE `month` WHEN 'Feb' THEN revenue END) Feb_Revenue,
SUM(CASE `month` WHEN 'Mar' THEN revenue END) Mar_Revenue,
SUM(CASE `month` WHEN 'Apr' THEN revenue END) Apr_Revenue,
SUM(CASE `month` WHEN 'May' THEN revenue END) May_Revenue,
SUM(CASE `month` WHEN 'Jun' THEN revenue END) Jun_Revenue,
SUM(CASE `month` WHEN 'Jul' THEN revenue END) Jul_Revenue,
SUM(CASE `month` WHEN 'Aug' THEN revenue END) Aug_Revenue,
SUM(CASE `month` WHEN 'Sep' THEN revenue END) Sep_Revenue,
SUM(CASE `month` WHEN 'Oct' THEN revenue END) Oct_Revenue,
SUM(CASE `month` WHEN 'Nov' THEN revenue END) Nov_Revenue,
SUM(CASE `month` WHEN 'Dec' THEN revenue END) Dec_Revenue
FROM Department
GROUP BY id;