Sorting of NULL Values
NULL values introduce a problem with sorting, and the various SQL products handle the ordering of NULL values in different ways. You should consult the relevant SQL manuals for more details. Four options exist:
MySQL treats NULL values as the lowest values in a column. Therefore, they are always placed at the bottom of the result if the order is descending and at the top if the order is ascending; see the following example and the accompanying result.
11. Get the different league numbers, and sort the result in descending order.