Question

假设我有一个类似于以下内容的表

CUSTOMER     PURCHASES
James        Basketball
James        Football
Alex         Basketball

我如何选择只购买篮球的客户？即只有亚历克斯。感谢您提供任何帮助或建议。

Answer 1

这是使用标准SQL的一种方式;

SELECT CUSTOMER FROM Table1
GROUP BY CUSTOMER
HAVING COUNT(*) = 1
  AND MAX(PURCHASES)='Basketball'

如果客户购买了多个篮球，您可以将COUNT(*)替换为COUNT(DISTINCT Purchases)。

如果你去了特定于数据库，一个更可读的选项可能是EXCEPT或MINUS，这里是一个SQL服务器版本;

SELECT CUSTOMER FROM Table1 WHERE PURCHASES='Basketball'
EXCEPT
SELECT CUSTOMER FROM Table1 WHERE PURCHASES<>'Basketball'

......这将与后者（可能购买不止一个篮球）的情况相同。

Answer 2

select * 
from purchase 
group by customer 
having count(*) = 1 
   and purchases = 'BasketBall';`

工作示例