I forgot to add the reason for the calculator ;
Code:
UPDATE lootdrop SET id=id-9 WHERE (id>=20 AND id<=30);
The highest old number was 10, and the lowest of the new numbers was 20
Highest old number(10) subtract from lowest of new numbers(20) = 10, then subtract one from your new number(10 - 1) to get 9.
You need a calculator for whan the numbers get big and ugly.