Implement SQL query with conditional sum and how does it work?

I am trying to write the SQL to generate the count of unique users who have purchased product B but have never purchased product C. user_id product date_purchased 1 A 2015-01-10 00:00:00.000 1 B 2014-11-23 00:00:00.000 1 C 2015-05-01 00:00:00.000…