I have read many Q&A's regarding NOT IN operator i.e If I use IN operator to filter NULL values and white spaces It is not working Why? and many others, But the problem is that when we use not in operator for non-primary key, it is not returning the response.
SELECT * FROM temp_customers WHERE temp_customers.fk_my_id NOT IN (SELECT fk_my_id FROM customers)
Where fk_my_id is non-primary key and might be a string.
Any Idea please?