T175
思路：
題目要求無論 person 是否有地址信息 都要提供
FirstName, LastName, City, State
所以可以使用外連接
答案：
select firstname,lastname,city,state from person p left join address a on p.personid=a.personid;
T176
思路：
首先想到使用排序和limit即可得到第二高的工資
select salary from employee order by salary desc limit 1,1
但是題目要求沒有時返回null
所以在外層加一個select
同時爲了避免工資相同的情況影響排序 所以要加上distinct
答案：
select (select distinct
salary from employee order by salary desc limit 1,1) secondhighestsalary;
答案中給出的另一種解法：
T181
select e1.name Employee from employee e1,employee e2 where e1.managerid=e2.id and e1.salary>e2.salary;
T182
select email from person group by email having count(*)>=2;
T183
select name customers from customers where id not in (select customerid from orders);
另一種方法：
select name customers from customers c left join orders o on c.id=o.customerid where o.id is null;
左外連接生成的表如下
T196
子查詢：delete from person where id not in (select id from(select min(id) id from person group by email) temp)
多嵌套一層select是因爲mysql中會出現1093錯誤
連接：
T197
select w1.id from weather w1,weather w2 where DateDiff(w1.recorddate,w2.recorddate)=1 and w1.temperature>w2.temperature;
使用DateDiff函數可以得到兩個日期之間的差 爲1則爲之後一天
T595
select name,population,area from world where population>25000000 or area>3000000;
T596
select class from courses group by class having count(distinct student)>=5;
Note:
學生在每個課中不應被重複計算
所以count條件爲distinct student
T620
select * from cinema where description != 'boring' and MOD(id,2)=1 order by rating desc;
T627
使用if
update salary set sex=if(sex='m','f','m');
使用case when
update salary set sex= case sex when 'f' then 'm' else 'f' end;