Leetcode 数据库板块，不收费的18道SQL练习题解题记录，GitHub
175 Combine Two Tables
# Write your MySQL query statement below.
SELECT
FirstName,
LastName,
City,
State
FROM
Person
LEFT OUTER JOIN Address ON Person.PersonId = Address.PersonId;176 Second Highest Salary
# Write your MySQL query statement below.
SELECT
IFNULL(
(
SELECT DISTINCT
Salary
FROM
Employee
ORDER BY
Salary DESC
LIMIT 1 OFFSET 1
),
NULL
) AS SecondHighestSalary;- 使用了
IFNULL(expr1, expr2)函数，将空值替换为NULL
177 Nth Highest Salary
CREATE FUNCTION getNthHighestSalary (N INT) RETURNS INT
BEGIN
DECLARE oset INT;
SET oset = N - 1;
RETURN (
# Write your MySQL query statement below.
SELECT
IFNULL(
(
SELECT DISTINCT
Salary
FROM
Employee
ORDER BY
Salary DESC
LIMIT oset, 1
),
NULL
)
);
END- limit后不能直接写N-1，因为limit后面不能接表达式；
178 Rank Scores
方法一：
# Write your MySQL query statement below
SELECT
Score,
(
SELECT
count(*)
FROM
(
SELECT DISTINCT
Score
FROM
Scores
) tmp
WHERE
tmp.Score >= Score
) As Rank
FROM
Scores
ORDER BY
Score DESC- 对每一个Score，找出>=它的score的数量，这个数量就是Rank；
方法二：
# Write your MySQL query statement below
SELECT
Scores.Score AS Score,
COUNT(*) AS Rank
FROM
Scores
JOIN (
SELECT DISTINCT
Score
FROM
Scores
) Ranking ON Scores.Score <= Ranking.Score
GROUP BY
Scores.Id
ORDER BY
Scores.Score DESC;- 求得每个Score与大于等于该Score的内连接表，再按照id分组，统计即可；
180. Consecutive Numbers
# Write your MySQL query statement below
SELECT DISTINCT
log1.Num AS ConsecutiveNums
FROM
LOGS log1,
LOGS log2,
LOGS log3
WHERE
log1.Id = log2.Id - 1
AND log2.Id = log3.Id - 1
AND log1.Num = log2.Num
AND log2.Num = log3.Num;- 自联结查询
SELECT DISTINCT log1.Num中的log1可替换为log2,log3；- 类似题目（更复杂）：601. Human Traffic of Stadium
181 Employees Earning More Than Their Managers
# Write your MySQL query statement below
SELECT
a.`Name` AS Employee
FROM
Employee AS a
INNER JOIN Employee AS b ON a.ManagerId = b.Id
AND a.Salary > b.Salary;182. Duplicate Emails
方式一：
# Write your MySQL query statement below
SELECT
Email
FROM
Person
GROUP BY
Email
HAVING
count(Email) > 1方式二：
# Write your MySQL query statement below
SELECT DISTINCT
a.Email
FROM
Person AS a
INNER JOIN Person AS b ON a.Id != b.Id
AND a.Email = b.Email;183. Customers Who Never Order
# Write your MySQL query statement below
SELECT
Customers. NAME AS 'Customers'
FROM
Customers
WHERE
Customers.Id NOT IN (
SELECT DISTINCT
CustomerId
FROM
Orders
);184 Department Highest Salary
SELECT
D.Name AS Department,
E.Name AS Employee,
E.Salary AS Salary
FROM
Employee E
INNER JOIN Department D ON E.DepartmentId = D.Id
AND (E.DepartmentId, E.Salary) IN (
SELECT
DepartmentId,
MAX(Salary) AS max
FROM
Employee
GROUP BY
DepartmentId
);- 将Employee表按照DepartmentId分组，再将各组中Salary最高的对应Id和该Salary组成一个集合，即各Department中最高Salary的集合；
再逐一判断employee的id和Salary是否在该集合中即可；
185 Department Top Three Salaries
# Write your MySQL query statement below
SELECT
d. NAME AS 'Department',
e1. NAME AS 'Employee',
e1.Salary
FROM
Employee e1
INNER JOIN Department d ON e1.DepartmentId = d.Id
WHERE
(
SELECT
COUNT(DISTINCT e2.Salary)
FROM
Employee e2
WHERE
e2.Salary > e1.Salary
AND e1.DepartmentId = e2.DepartmentId
) < 3
ORDER BY
Department;- 工资前三名的员工，可理解为：在该部门中比这三名员工中任一位工资高的人不超过3人，据此便可作为WHERE条件，加上 DepartmentId 连接，便可找出一个 Department 中工资前三的员工记录，最后再与 Department 表内连接，列出部门名称即可；
196 Delete Duplicate Emails
# Write your MySQL query statement below
DELETE p1
FROM
Person p1,
Person p2
WHERE
p1.Email = p2.Email
AND p1.Id > p2.Id;197. Rising Temperature
# Write your MySQL query statement below
SELECT
w1.Id
FROM
Weather w1
INNER JOIN Weather w2 ON DATEDIFF(w1.Date, w2.Date) = 1
AND w1.Temperature > w2.Temperature;- 使用
DATEDIFF(expr1, expr2)函数
262. Trips and Users
# Write your MySQL query statement below
SELECT
t.Request_at AS `Day`,
ROUND(
SUM(IF (t.Status = 'completed', 0, 1)) / COUNT(*), 2
) AS `Cancellation Rate`
FROM
Trips t
INNER JOIN Users u ON t.Client_Id = u.Users_Id
WHERE
t.Request_at BETWEEN '2013-10-01'
AND '2013-10-03'
AND u.Banned = 'No'
AND u.Role = 'client'
GROUP BY
t.Request_at
ORDER BY
t.Request_at;- 先筛选出符合条件的 trip 记录，然后按日期进行分组，再分别计算各组的 Cancellation Rate；
- 计算 Cancellation Rate 时，使用
IF(expr1,expr2,expr3)函数、ROUND()函数、sum()函数；
595 Big Countries
# Write your MySQL query statement below
SELECT
NAME,
population,
area
FROM
World
WHERE
area > 3000000
OR population > 25000000;
596 Classes More Than 5 Students
# Write your MySQL query statement below
SELECT
class
FROM
courses
GROUP BY
class
HAVING
COUNT(DISTINCT student) >= 5;WHERE子句用于筛选行记录，筛选分组要使用HAVING子句；
601 Human Traffic of Stadium
# Write your MySQL query statement below
SELECT DISTINCT
t1.*
FROM
stadium t1,
stadium t2,
stadium t3
WHERE
t1.people >= 100
AND t2.people >= 100
AND t3.people >= 100
AND (
(
t3.id - t2.id = 1
AND t2.id - t1.id = 1
AND t3.id - t1.id = 2
) -- id: t1 < t2 < t3
OR (
t2.id - t1.id = 1
AND t2.id - t3.id = 2
AND t1.id - t3.id = 1
) -- id: t3 < t1 < t2
OR (
t1.id - t2.id = 1
AND t1.id - t3.id = 2
AND t2.id - t3.id = 1
) -- id: t3 < t2 < t1
)
ORDER BY
t1.id;- 自联结查询
- 由于需要列出匹配的所有记录，因此此处的t1需能匹配到第一天、第二天、第三天……直到连续最后一天的所有情况；
- 若
SELECT DISTINCT t1.*中的t1替换成tx，则 WHERE 子句中的条件需要相应地做出改变； - 类似题目（简单）：180. Consecutive Numbers
620 Not Boring Movies
# Write your MySQL query statement below
SELECT
*
FROM
cinema
WHERE
MOD (id, 2) != 0
AND description != 'boring'
ORDER BY
rating DESC;627 Swap Salary
# Write your MySQL query statement below
UPDATE salary
SET
sex = CASE sex
WHEN 'm' THEN 'f'
ELSE 'm'
END;- 使用
CASE WHEN流程控制