Background: I inherited an excel workbook detailing # of units sold each month of a given year by customer, each tab is the year with active customers names in Col. A, and Jan-Dec in Col. B-M, w/ a YTD total in Col. N, & the historical YTD data of the previous year in Col. O. Only the active customers for that year are included on each tab in Col. A, which means not everyone on the current year's worksheet may be listed in the previous year's, also there are definitely customers listed on the previous year that are inactive so far for this year, so I can't just sum the same cells across each worksheet because they don't match up.
I basically need to create a formula in Col. O that says "if the customer's name listed on worksheet '2020' in Column A matches the customer's name listed on worksheet '2019' in Column A, add the corresponding row for that customer including the data listed in columns (in this case, since I'm looking for 2019 Jan. - Jul. data) B-H of that row, if no match leave it blank"
Unfortunately I think I've exceeded my slightly above average excel knowledge database on this one. It seems like I'd need a sumif formula, but I think I needs to be more robust than that. I will admit to being limited on my macros knowledge, so take it easy on me and explain it as if I was an idiot please 
