List the continents that have a total population of at least 100 million.
select continent from world group by continent having sum(population)>=100000000
这题考察的是使用集聚函数生成表之后，如何过滤
一般我们生成一个查询结果集都会使用 where 表达式来过滤不想要的内容，
但是group by分组表达式在SQL中是在where过滤之后才能执行，所以当group by分组之后，我们需要另外一个表达式having
来过滤分组聚集后的结果集
having sum(population)>=100000000
Wrong answer. Too many rows
| continent |
|---|
| Africa |
| Asia |
| Caribbean |
| Eurasia |
| Europe |
| North America |
| Oceania |
| South America |