Update If Matching Data (Maybe No PK)

Basically without going into too much detail, our company gets databases arriving and put onto our systems which have been made my other organizations with no guarantee of what the primary key is or if this is one at all.

I should probably give my main problem in an example for clarity:

Currently I have a .csv file full of data that needs to be put into say TableA. However I do not know if TableA has a primary key or not, or if the file that needs to be inserted into TableA contains duplicate data. I have the importer sorted that does this if you ignore the problem of duplicate data, however what I would like is an MS SQL query that does the following (but I cannot figure it out):

Assuming we are reading through the file line-by-line and a check is performed each time:

1.If there is a line with a primary key in the file that matches a primary key in TableA in the database update that row in the database with the line in the file.

2. If there is no primary key on the table and there is an exact data match between the line in the file and a row in the database then update it.

3. If neither 1 or 2 are successful then just insert the data.

Obviously the potential lack of a PK here makes things a lot more convoluted.