I see no indication that you have even attempted to solve this problem. Also the problem definition, sparse as it is, makes question 1, if I understand how OBJ_ID is generated, invalid as you can't have the same OBJ_ID occurring more than once in this table (the 'monotonically increasing number' definition, which means a sequence). Of course your description of the problem could be wrong.
If you want help then you need to do the following:
1) Provide a COMPLETE description of the problem, including setup [table creation and population].
2) Make an attempt to solve this yourself; provide your code and any output you receive
3) Do NOT ask US to do your homework for you. We won't, plainly and simply put.
David Fitzjarrell