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;