Question : comparing two tables based on two identical fields

Tables:  Details, Index
Fields:  FileName, FileDate

What I would like to do is step through each record in Details, when the FileName data is identical between the two tables, the word "Present" is stored in Index.FileStatus and moves onto the next record to check.  When Details has a FileName that doesn't compare to Index, the word "Missing" is stored in Index.FileStatus and moves onto the next record to check.  When Index has a FileName that doesn't compare to Details, then that FileName, FileDate, and AppDetails (query) is run and moves onto the next record to check.  

I hope this is enough information. I appreciate any help you can provide.  I have a deadline tomorrow and this is the last piece to implement into the rest of my code and test.  Thank you.

Answer : comparing two tables based on two identical fields

Using a query is the simplest approach to the first part of this.
I don't understand the third bit and the second bit doesn't make sense.

QrySetPresent:

Update [Index] inner join [Details]
on Index.filename = Details.Filename
Set Index.Filestatus = "Present"

You then say 'When Details has a FileName that doesn't compare to Index, the word "Missing" is stored in Index.FileStatus'
So you want to set Index.FileStatus to 'Missing' if the record is not present in Index.  So which record in Index would you be setting to this value?

As I said I don't understand what you want to do in the third step.