Thanks for article and comments - I learnt a lot.

In MySQL 😀 this is the fastest by far:

SELECT

CustomerId

FROM Purchase

WHERE ProductCode IN ('A','B', 'C')

GROUP BY CustomerID

having sum(case when ProductCode = 'A' then 1 else 0 end) > 0

and sum(case when ProductCode = 'B' then 1 else 0 end) > 0

and sum(case when ProductCode = 'C' then 1 else 0 end) = 0

Unfortunately MySQL does not have the EXCEPT operator.

-jj