[redhale]

Forget HAVING -- in my opinion RANK is the most useful SQL function that barely anyone knows about. Has gotten me out of more sticky query issues than I can count.

[jupiter909]

The entire family of Windowing Functions which are mostly ubiquitous on all modern SQL seem to be glossed over by most.

[mike256]

RANK is a good example of a useful less known window function. Please also consider dense_rank and row_number. All 3 have their advantages and disadvantages... (Rank leaves holes after rows with the same order, dense_rank gives consecutive numbers, row_number just numbers the rows.)