Data Pairing

Hi,

Sorry to bother anyone but I'm working on a project for college at the moment and just can't get this problem to work out.
I've two excel documents - one with two columns: user id and account no.
The other has user id and no. of items purchased. there is over 10,000 users on the files but not all users have purchased items so ive gaps in the purchased items list.

I've copied the information so that it is all in one spreadsheet now but I need to find a way to pair the data up based on the user ids?

I would really appreciate any help.... here's a small sample below of what i mean
book 1 book 2
USER ID USER ID ITEMS BOUGHT

user 1 user 2 3
user 2 user3 20
user 3 user4 11
user 4 user5 4
user 5 user 7 25
user 6 user 9 4
user 7
user 8
user 9

Do you need total Items bought?
Add a column to the sheet that has the ID and account number: =Sumifs(SumRange,CriteriaRange,Criteria)
SumRange being the column on sheet 2 that has the units sold
Criteria Range being the column with the id on sheet 2
Criteria being = A1 (A is the column on current sheet with the id) 1(is the CURRENT Row)

Could you explain me what is (look_up value,table_array,col_index_num,

I guess range_lookup is data where I am looking for the matching number

Could you explain me what is (look_up value,table_array,col_index_num,

I guess range_lookup is data where I am looking for the matching number