the used table is here:

<http://sqlzoo.net/wiki/SUM_and_COUNT>

There's a thing I can't understand. If I use this:

```
select continent, count(name)
from world
group by continent
having population >= 10000000 

```

They say Unknown column 'population' in 'having clause'. But this other query is considered fine:

```
select continent
from world
group by continent
having sum(population) >= 100000000

```

Why? Why here the population column is recognized and on the first query is not?