可以将条件添加到子查询：

SELECT t1.customer, t1.type, t1.product
FROM yourTable t1
WHERE EXISTS (SELECT 1
              FROM yourTable t2
              WHERE t2.customer = t1.customer AND
                    t2.type = 'a' AND
                    t2.product = 1
             );

如果您只想要客户：

select customer
from yourtable
group by customer
having sum(case when type = 'a' and product = 1 then 1 else 0 end) > 0;

这是您要求的版本

WITH CTE_yourTable AS
(
SELECT CUSTOMER
FROM yourTable
WHERE 
type = 'a' AND
product = 1)
SELECT t1.customer, t1.type, t1.product
FROM yourTable t1
WHERE EXISTS (SELECT 1
              FROM CTE_yourTable t2
              WHERE t2.customer = t1.customer 
             );