LeetCode sql语句 学习专题【Easy】
sql语句写的不多，希望提高一下自己的sql能力，按照Easy->medium->hard的顺序练习
题目链接如下： https://leetcode.com/problemset/database/
目录
595. Big Countries
627. Swap Salary
620. Not Boring Movies
182. Duplicate Emails
175. Combine Two Tables
181. Employees Earning More Than Their Managers
183. Customers Who Never Order
596. Classes More Than 5 Students
197. Rising Temperature
196. Delete Duplicate Emails
176. Second Highest Salary
595. Big Countries
找出一张表中满足条件的记录，考察or的用法
# Write your MySQL query statement below
select name,population,area from World where area>=3000000 or population>=25000000;627. Swap Salary
将表中的字段按照要求转换为别的，需要用到 sql里的判断 CASE X WHEN .. THEN .. ELSE ...END;
# Write your MySQL query statement below
update salary
set sex=CASE sex WHEN 'f' THEN 'm'
ELSE 'f' END;
620. Not Boring Movies
找出满足条件，且按照某字段排序的记录
# Write your MySQL query statement below
select * from (
select * from cinema where description !='boring' and id %2=1
) A order by rating DESC
182. Duplicate Emails
表中有重复的字段，按照要求统计出来
# Write your MySQL query statement below
select Email from(
select Email,count(Email) as num from Person
group by Email
)A where num>1
175. Combine Two Tables
根据某个字段连接两张表，使用left join
# Write your MySQL query statement below
select p.FirstName, p.LastName, a.City, a.State from Person p
left join Address a
on p.PersonId = a.PersonId ;
181. Employees Earning More Than Their Managers
根据要求找到满足的字段，在同一张表中有以来关系，可以采用 tableName a的形式指代一张表
# Write your MySQL query statement below
select a.Name as Employee from
Employee as a,Employee as b
where
a.ManagerId = b.Id and
a.Salary > b.Salary
183. Customers Who Never Order
两张表，查询不在另外一张表里的内容，需要使用not in
# Write your MySQL query statement below
select Name as Customers from
Customers as a
where
a.id not in(
select CustomerId from Orders
)
596. Classes More Than 5 Students
找出人数超过5个人的课程，注意可能有重复字段 需要用DISTINCT
# Write your MySQL query statement below
select class from (
select class,count(DISTINCT student) as num from courses group by class
) A where num>=5
197. Rising Temperature
需要找出比前一天温度高的记录，需要使用函数datadiff()
# Write your MySQL query statement below
select b.Id from Weather as a,Weather as b
where datediff(b.RecordDate,a.RecordDate)=1 and a.Temperature < b.Temperature
SELECT
weather.id AS 'Id'
FROM
weather
JOIN
weather w ON DATEDIFF(weather.date, w.date) = 1
AND weather.Temperature > w.Temperature
;
196. Delete Duplicate Emails
删除重复的记录保留主键最小的那个
# Write your MySQL query statement below
delete p1 from Person p1,Person p2
where p1.Email = p2.Email and p1.Id > p2.Id
176. Second Highest Salary
查询第二高的价格，注意 1价格可能重复 2.可能没有
# Write your MySQL query statement below
select (
select distinct Salary from Employee order by Salary DESC limit 1,1
) as SecondHighestSalary