SELECT CustomerID
	FROM #Purchase
	WHERE ProductCode = 'A'
	INTERSECT
	SELECT CustomerID
	FROM #Purchase
	WHERE ProductCode = 'B'
	EXCEPT 
	SELECT CustomerID
	FROM #Purchase
	WHERE ProductCode = 'C'

;WITH CTE AS
(
	SELECT CustomerID
	FROM #Purchase
	WHERE ProductCode = 'A'
	GROUP BY CustomerID
)
,	CTE2 AS
(
	SELECT CustomerID
	FROM #Purchase
	WHERE ProductCode = 'B'
	GROUP BY CustomerID
)
,	CTE3 AS
(
	SELECT CustomerID
	FROM #Purchase
	WHERE ProductCode = 'C'
	GROUP BY CustomerID
	
)
select CTE.CustomerID from CTE
left join CTE3 ON
	CTE.CustomerID = CTE3.CustomerID
where CTE.CustomerID in (SELECT CustomerID FROM CTE2)
AND CTE3.CustomerID IS NULL
GROUP BY CTE.CustomerID

     Jul 13  

Sometimes this is me

SELECT DISTINCT
	CustomerID
FROM #Purchase p1
WHERE ProductCode = 'A'
	AND EXISTS (SELECT
					1
				FROM #Purchase p2
				WHERE p2.CustomerID = p1.CustomerID
					AND p2.ProductCode = 'B')
	AND NOT EXISTS (SELECT
						1
					FROM #Purchase p3
					WHERE p3.CustomerID = p1.CustomerID
						AND p3.ProductCode = 'C')

Table 'Worktable'. Scan count 49955, logical reads 151393, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 655 ms,  elapsed time = 739 ms.

Table 'Worktable'. Scan count 3, logical reads 2156, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 406 ms,  elapsed time = 478 ms.

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

SELECT
	b.CustomerID
FROM
	(
	SELECT
		a.CustomerID
	FROM
		(
		SELECT
			DISTINCT
			CustomerID,
			Product
		FROM
			Purchase
		WHERE
			Product IN ('A','B')) AS a
	GROUP BY
		a.CustomerID
	HAVING
		COUNT(Product) = 2
	) AS b LEFT OUTER JOIN
	Purchase p ON b.CustomerID = p.CustomerID AND p.Product = 'C'
WHERE
	p.Product IS NULL