1.https://leetcode.com/problems/big-countries/
最简单的有条件的筛选：
select name, population, area from World where area > 3000000 or population > 25000000
2.https://leetcode.com/problems/swap-salary/:swap列值的属性：
考查update或select的用法，swap这个if函数，有点类似C语言里面的“?:”操作
select id, name, if(sex = 'm','f','m') as sex, salary from salary
或update salary set sex=CHAR(ASCII('f')^ASCII('m')^ASCII(sex));#很妙的抑或操作，但是容易写错
3.https://leetcode.com/problems/not-boring-movies 考查排序操作order by（默认升序，逆序加desc）
select * from cinema where id % 2 and description != 'boring'
order by rating desc
4.https://leetcode.com/problems/duplicate-emails/ 考查去重操作：
a.GROUP BY 语句用于结合合计函数，根据一个或多个列对结果集进行分组。
b.having是分组（group by）后的筛选条件，分组后的数据组内再筛选
c.count:计数
d.distinct:去重
select distinct Email from Person where Email in (select Email from Person group by Email having count(Email) > 1)
5.https://leetcode.com/problems/combine-two-tables/ 考查两个表的join，又因为有“for each person in the Person table, regardless if there is an address for each of those people:” 可知考查的是left join。
select Person.FirstName, Person.Lastname, Address.City, Address.State
from Person left join Address
on Person.PersonId = Address.PersonId
6.https://leetcode.com/problems/employees-earning-more-than-their-managers/：考查同一个表的不同实例，通过join操作建立联系。小细节：最后table的列名发生了变化。
select e1.Name as Employee from Employee e1
join Employee e2 on e1.ManagerId = e2.Id
where e1.Salary > e2.Salary
7.https://leetcode.com/problems/customers-who-never-order/：考查left join操作（但是是差集），或者not in操作
ed0:
select Customers.Name as Customers
from Customers left join Orders
on Customers.Id = Orders.CustomerId where Orders.Id is null
ed1:
select Customers.Name as Customers from Customers
where Customers.Id not in (select CustomerId from Orders)
8.https://leetcode.com/problems/classes-more-than-5-students/:考查group by
select class from courses group by class
having count(distinct(student)) >= 5
9.https://leetcode.com/problems/rising-temperature/ 依然是考查两个实例用法，还有时间差的用法 datediff
select e1.Id from Weather e1, Weather e2
where datediff(e1.RecordDate, e2.RecordDate) = 1 and e1.Temperature > e2.Temperature
10.https://leetcode.com/problems/delete-duplicate-emails/ 删除重复邮箱，保留较小编号。考查delete用法。
delete p2 from Person p1, Person p2
where p1.Email = p2.Email and p1.Id < p2.Id
11.https://leetcode.com/problems/second-highest-salary/：第二高工资
a。最直观的做法，先select出最高的，然后再找出小于最大值的 最大值
select max(Salary) as SecondHighestSalary
from Employee
where salary < (select max(Salary) from Employee)
b。引入排序。复杂篇，下次再写。