MySQL组加入空值

Question

我正在尝试创建一个 MySQL 查询，该查询将返回与某个日期范围内创建的用户相关的客户数量。我有以下查询:

SELECT COUNT(`customer`.`user_id`) as `count`, `user`.`first_name`,  `user`.`last_name`
FROM (`user`) 
LEFT JOIN `customer` ON `user`.`id` = `customer`.`user_id`
WHERE `customer`.`date_created` >= '2014-03-27 23:41:28' AND `customer`.`date_created` <= '2014-04-04 00:41:28'
GROUP BY `customer`.`user_id` 
ORDER BY `count`
LIMIT 5

如果我删除 where 子句，那么我会得到我期望的结果，包括空值:

count   first_name   last_name
0       Name         Name
1       Name         Name
3       Name         Name
10      Name         Name

问题是，当我引入 where 子句时，我没有得到没有计数值的返回行。谁能帮忙？

Answer 1

将您的 where 条件移动到 ON() 子句，这样它将过滤来自右表的结果而不是所有结果集，并且将为来自左表的不匹配结果产生 0 计数

SELECT COUNT(`customer`.`user_id`) as `count`, 
`user`.`first_name`, 
 `user`.`last_name`
FROM `user` 
LEFT JOIN `customer` ON (
`user`.`id` = `customer`.`user_id`
AND `customer`.`date_created` >= '2014-03-27 23:41:28'
 AND `customer`.`date_created` <= '2014-04-04 00:41:28')
GROUP BY `user`.`id` 
ORDER BY `count`
LIMIT 5