Hi Abu, thank you for the kind mention - I take it that phase of the project is now concluded.
Have a try with this. It works by first selecting retained rows which don't get a mention in discarded rows, then left joining to self. Hope that makes sense 🙂
SELECT a.*
FROM testing a
LEFT JOIN testing b ON b.retained = a.dropped
WHERE NOT EXISTS (SELECT 1 FROM testing i WHERE a.retained = i.dropped)
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden