Mysql 内部联接不返回任何行

我试图从数据库中获取2015年4月没有任何事件的客户名称：

SELECT customer.customerId, customer.Name, event.Date
FROM customer
INNER JOIN event ON event.customerId=customer.customerId
INNER JOIN 
    (
    SELECT eventId AS evid, customerId
    FROM  event
    WHERE year(event.Date)=2015 AND month(event.Date)=04
    GROUP BY customerId
    )
EV ON event.eventId = EV.evid
WHERE event.customerId IS NULL

SELECT customer.customerId, customer.Name
FROM customer
WHERE customerId NOT IN
(
    SELECT customerId
    FROM  event
    WHERE year(event.Date)=2015 AND month(event.Date)=04
) EV;

---

我得到零行，我应该有大约20个客户的名字在那里。查询有什么问题？

您应该使用左联接，并且只需要在表中联接一次。但是，您不能选择活动日期，因为您获得的客户在此期间没有任何活动：

select
  customer.customerId, customer.Name
from
  customer
  left join event on event.customerId = customer.customerId
    and year(event.Date) = 2015 and month(event.Date) = 4
where
  event.customerId is null

---

这将为您提供2015年4月没有任何活动的所有客户的客户id和姓名：

SELECT customer.customerId, customer.Name, event.Date
FROM customer
INNER JOIN event ON event.customerId=customer.customerId
INNER JOIN 
    (
    SELECT eventId AS evid, customerId
    FROM  event
    WHERE year(event.Date)=2015 AND month(event.Date)=04
    GROUP BY customerId
    )
EV ON event.eventId = EV.evid
WHERE event.customerId IS NULL

SELECT customer.customerId, customer.Name
FROM customer
WHERE customerId NOT IN
(
    SELECT customerId
    FROM  event
    WHERE year(event.Date)=2015 AND month(event.Date)=04
) EV;

---

为什么在内部联接之后需要空Id？