MySQL Learning Note (it performs the steps)

The basic steps are: (Not very accurate, please read this article)
1.from
2.join on
3.where
4.group by
5.having
6.order by
7.select
8.distinct, Sum,...
9.limit

Join table must have an index or the Cartesian product.
INNER JOIN's Special
Correctly speaking, MySQL will first select the best table join table method with the filter condition of the base on where. Then it is the for 1st table that will filter where to start the join first.
However, we usually use the straight join to control it, lest we don't know that the table will be in the final join.
Having is filtering out the rows after group by, this time the filter can use the count sum of those, which is not used in the where.