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This is a common request and it usually indicates an error in database design. Since these values can be compared they most likely represent the same attribute and should all be stored under the same field name (but different rows) in the database.
Because of this concept there aren't good built-in tools (in SQL for example) to handle this request.
Access provides the ability to VBA functions in SQL so you can use the IIF() function to compare two values and take action based on a comparison:
You could use a UNION query to do this:
SELECT TOP 1 FROM
(SELECT MAX(A) FROM yourTable
SELECT MAX(B) FROM yourTable
SELECT MAX(C) FROM yourTable
SELECT MAX(D) FROM yourTable
) ORDER BY 1