Question

您好我有一个优惠券系统，它使用一个表（称为“优惠券”）来存储关于可以生成的每种类型的可用优惠券的信息，以及另一个表（生成的优惠券）来存储关于生成的每个优惠券的信息。我很难比较每个表格中的信息。

模式：

table: coupons
+----+------+--------------------+---------------+
|  id| owner|     expiration_date| limit_per_user|
|  15|    34| 2011-09-18 00:00:00|              2|
+----+------+--------------------+---------------+

table: generatedcoupons
+----+----------+------+--------------------+------+--------+
|  id| coupon_id| owner|                date|  used| user_id|
|   1|        15|    34| 2011-09-17 00:00:00| false|     233|
+----+----------+------+--------------------+------+--------+

我正在尝试运行查询以从用户的角度显示优惠券（即所有查询都有where user_id='$userid'。我无法弄清楚如何显示limit_per_user尚未显示的所有优惠券遇见......这就是我所得到的不起作用：

select * 
from coupons 
where owner=34 
and (count(SELECT * from generatedcoupons where user_id=233 and coupon_id=coupons.id)<limit_per_user)

Answer 1

对于MySQL，加入通常比子查询更快，更可靠，但会让你觉得有点困难。在这种情况下，您需要根据连接的行数限制，这需要进行后分组计算;幸运的是，这正是HAVING条款的用途：

select coupons.*
from coupons
left join generatedcoupons on user_id = 233 and coupon_id = coupons.id
where coupons.owner = 34
group by coupons.id
having count(generatedcoupons.id) < limit_per_user

Answer 2

select * 
from coupons as c left join
generatedcoupons as g on g.user_id = 233 and g.coupon_id = c.id
where c.owner=34
group by c.id
having count(g.id) < c.limit_per_user

Mysql嵌套查询将一个值传递给嵌套查询

2 个答案: