你可以加入 CUSTOMER 以及 ORDERS 包含特定订单的表 <chemno> 使用只购买产品的custnum的子查询：

SELECT
    CUSTNUM, NAME 
FROM
    CUSTOMER c
INNER JOIN
    ORDERS o ON o.CUSTNUM = c.CUSTNUM and o.CHEMNO = <chemno>
INNER JOIN
    ( SELECT
          CUSTNUM 
      FROM
          ORDERS
      GROUP BY
          CUSTNUM 
      HAVING
          COUNT(DISTINCT CHEMNO) = 1 ) t ON t.CUSTNUM = o.CUSTNUM

我会用一个来解决这个问题 join 在两个表之间，然后按列分组 CUSTNUM 的 ORDERS 最后在 HAVING 子句，如下所示：

SELECT
    o.CUSTNUM,
    c.NAME
FROM
    ORDERS AS o
INNER JOIN
    CUSTOMER AS c ON c.CUSTNUM = o.CUSTNUM
GROUP BY
    o.CUSTNUM
HAVING
    ( COUNT(DISTINCT o.CHEMNO) = 1 AND MIN(o.CHEMNO) = <some_chemno> )

好吧，慢一天。。。

SELECT DISTINCT x.custnum 
  FROM orders x 
  LEFT 
  JOIN orders y 
    ON y.custnum = x.custnum 
   AND y.chemno <> x.chemno 
 WHERE x.chemno = 9377 
   AND y.order_id IS NULL;

这个任务的其余部分留给读者作为练习