算法刷多了，感覺腦子里全是漿糊了，所以做一做數據庫的題調整一下，哈哈哈哈！
175. Combine Two Tables
這道題就是一道簡單的左外鏈接啦，不過犯了一個大錯誤就是，左外鏈接的鏈接條件不是寫在where里，是在關鍵詞on后面的！
select FirstName,LastName,City,State from Person left outer join Address on Person.PersonId = Address.PersonId;
176. Second Highest Salary
尋找數組中第二大的數，我們只要在選擇的時候去掉最大的數就好，所以我們使用一個子查詢來解決：
select max(Salary) as SecondHighestSalary from Employee where Salary < (select max(Salary) from Employee)；
181. Employees Earning More Than Their Managers
一開始拼命的想，用子查詢的話，怎么把managerid傳到子查詢中呢？后來豁然開朗，自鏈接不就好了嘛！
select e1.Name as Employee from Employee as e1,Employee as e2 where e1.ManagerId=e2.Id and e1.Salary > e2.Salary
182. Duplicate Emails
思路簡單，用groupby分組，然后用count計數，最后用having篩選
select Email from Person group by Email having count(*) > 1
183. Customers Who Never Order
in關鍵詞的使用，使用一個子查詢查找出所有在Orders表中出現過的CustomersId即可。
select Name as Customers from Customers where id not in (select distinct CustomerId from Orders);
196. Delete Duplicate Emails
兩個表進行自鏈接，然后只留下id相同的部分，去掉id不同的部分，思路太神奇：
delete p1 from Person as p1,Person as p2 where p1.Email = p2.Email and p1.Id > p2.Id
197. Rising Temperature
這里要用到mysql的日期處理函數(http://www.cnblogs.com/ggjucheng/p/3352280.html),使用to_days函數將日期轉換為整數。
select w2.Id from Weather w1,Weather w2 where To_days(w1.Date)+1 = to_days(w2.Date) and w1.Temperature < w2.Temperature
595. Big Countries
簡單的select
select name,population,area from World where area > 3000000 or population > 25000000
596. Classes More Than 5 Students
要注意的是，這里的count的時候要加distinct？我覺得是胡扯。
select class from courses group by class having count(distinct student) >= 5
620. Not Boring Movies
簡單的應用排序：
select * from cinema where (id % 2) = 1 and description <> 'boring' order by rating desc
627. Swap Salary
這道題應該算簡單題里面最簡單的了吧，還真沒想到，不過看了答案感覺豁然開朗，用了二進制運算，兩個相同字母的二進制異或是0.
update salary set sex = CHAR(ASCII('f') ^ ASCII('m') ^ ASCII(sex));
也可以使用case when，第一次接觸，還不太熟悉，感覺mysql還是很強大的。
update salary set sex = case when sex ='m' then 'f' else 'm' end
