1. select countrycode, avg(population) from world.city group by countrycode having avg(population)>=100000;
2. select countrycode, avg(population) from world.city where population>=100000 group by countrycode;
3. select a.district from (select district, country from world.city where population>=100000) as a;
4. select name,ID, (select name from world.country world.country.code = world.countrycode) worle_name from world.city;
5.select a.id,a.name,b.name, b.gnp from
(select id,name,countrycode from world.city where population>100000) as a
inner join #还可以left join,right join,但返回值有区别
(select code,name,gnp from world.country) as b
on a.countrycode = b.code;
6. select countrycode from world.city
union
select code from world.code;
#alter table world.country modify column population bigint; 修改数据格式
7. use world;
create table if not exists world.xiaoming_test1 as
select a.*,b.*from #注意是否有重名，如有，要重命名
(select * from world.city) as a
left join
(select * from world.country) as b
on a.countrycode = b.code;
left join
(select * from world.language) as c
on a.countrycode = c.countrycode;
8. drop talbe if exists world.xiaoming_test1;
9. create table world.xiaoming_test2
(id int,
countrycode char(5),
population int,
gnp float,
language char(15),
priamary key(id) #主键不能重复,如有，换或者多个，或没有主键
);
10. Insert table world.xiaoming_test2
select a.id,a.name as name_a, a.countrycode, c.gnp, c.language #注意是否有重名，如有，要重命名
(select * from world.city) as a
left join
(select * from world.country) as b
on a.countrycode = b.code;
11. alter table
left join
(select * from world.language) as c
on a.countrycode = c.countrycode;
12. alter table world.xiaoming_test2
add new_column char(15);
13. alter table world.xiaoming_test2
drop gnp;
14. update table world.xiaoming_test2
set population = 1
where gnp is null;
