Mysql嵌套查询传递一个值嵌套查询

Question

您好我有一个优惠券系统，使用一个表（称为“优惠券”）存储有关每种可用的优惠券，可以生成的信息和另一个表（generatedcoupons）存储有关每个优惠券的信息。我很难比较每张表中的信息。

的模式：

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| 
+----+----------+------+--------------------+------+--------+ 

我试图运行查询，从用户的观点（即所有的查询将有where user_id='$userid'点显示优惠券我无法弄清楚如何显示所有。优惠券在limit_per_user没有得到满足......这里是我有什么不工作：

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，您需要根据加入的ro的数量进行限制ws，这需要后分组计算;幸运的是，这恰恰是HAVING条款是什么：

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