题目链接：https://sqlzoo.net/wiki/SUM_and_COUNT
1
SELECT SUM(population) FROM world
2
select distinct continent from world;
3
select sum(gdp) from world
where continent='Africa';
4
select count(name) from world
where area >= 1000000;
5
select sum(population) from world
where name in ('Estonia', 'Latvia', 'Lithuania')
6
select continent ,count(name) from world
group by continent;
7
select continent,count(name) from world
where population >= 10000000
group by continent;
8
select continent from world
group by continent
having sum(population) >= 100000000;
