Question

此查询可能出现问题：

select count(customer_email) as num_prev
  from _pj_cust_email_by_date
  where order_date < '2011-02'
  and customer_email is not null
  group by customer_email having count(order_date) > 0;

返回行结果，例如：

当我尝试获取完整计数时，指定在指定日期范围内购买的客户总数

_pj_cust_email_by_date是一个视图，仅返回YYYY-MM-DD格式的电子邮件地址和订单日期。我无权使用除此视图之外的任何内容。

Answer 1

GROUP BY导致了这一点。

它会导致每个组返回一个结果行，对于每个不同的customer_email值，都会返回此结果。

如果您需要不同电子邮件地址的总数，则需要删除GROUP BY子句并将COUNT更改为COUNT(DISTINCT customer_email)。

Answer 2

您需要进一步查询

select count(*) CustomerCount
from (
    select count(customer_email) as num_prev
    from _pj_cust_email_by_date
    where order_date < '2011-02'
    and customer_email is not null
    group by customer_email having count(order_date) > 0;
) as innercount

这通常是方法，但由于您使用的是having count(order_date) > 0，我认为您只需要

select count(distinct customer_email) as num_prev
from _pj_cust_email_by_date
where order_date < '2011-02' and customer_email is not null

因为HAVING子句永远不会用空的order_dates来详细说明，这实际上会使HAVING子句成为dud 。

MySQL count（）查询返回行而不是total？

2 个答案: