MySQL，子查询返回null Date和Datetime

Question

我有一个奇怪的问题，我有子查询，应该在给定的日子里返回有关用户假期的信息

    SELECT `User`.`id`, `User`.`f_name`, `User`.`l_name`, `User`.`discharge`, `HourCard`.*, SUBSTRING(HourCard.date, 1, 10),
     (SELECT HOUR.date <---- subquery
      FROM hours AS HOUR
      WHERE DATE(HOUR.date) = DATE(HourCard.date) AND Hour.subordinate_id = User.id AND Hour.day_off_id IS NOT NULL) AS info

    FROM `kadry`.`hour_cards` AS `HourCard`
    LEFT JOIN `kadry`.`users` AS `User` ON (`HourCard`.`subordinate_id` = `User`.`id`)
    WHERE `User`.`assigned` = 'HHDO1461932902' AND `User`.`active` = 1 AND DATE(`HourCard`.`date`) BETWEEN '2016-06-01' AND '2016-06-30'
    ORDER BY `User`.`l_name` ASC, `HourCard`.`date` ASC

查询返回：

    | id | f_name | l_name | discharge | id | subordinate_id | date | date_type | readed | SUBSTRING(HourCard.date, 1, 10) | info | 
    | 00623 | john | doe | \null | 446610 | 00623 | 2016-06-01 17:06:12 | 1 | 0 | 2016-06-01 | \null | 
    | 00623 | john | doe | \null | 447779 | 00623 | 2016-06-02 08:00:49 | 0 | 0 | 2016-06-02 | \null | 
    | 00623 | john | doe | \null | 447778 | 00623 | 2016-06-02 17:13:53 | 1 | 0 | 2016-06-02 | \null | 
    | 00623 | john | doe | \null | 454696 | 00623 | 2016-06-03 08:03:40 | 0 | 0 | 2016-06-03 | \null | 
    | 00623 | john | doe | \null | 454697 | 00623 | 2016-06-03 17:03:36 | 1 | 0 | 2016-06-03 | \null | 
    | 00623 | john | doe | \null | 454699 | 00623 | 2016-06-04 08:05:07 | 0 | 0 | 2016-06-04 | \null | 

但是当我将子查询更改为此

时

(
    SELECT HOUR.date
    FROM hours AS HOUR
    WHERE  Hour.date = '2016-06-01' AND Hour.subordinate_id = User.id AND Hour.day_off_id IS NOT NULL) AS info

这个查询会返回这样的好结果：

    | id | f_name | l_name | discharge | id | subordinate_id | date | date_type | readed | SUBSTRING(HourCard.date, 1, 10) | info | 
    | 02979 | johny | notDoe | \null | 455888 | 02979 | 2016-06-06 06:49:32 | 0 | 0 | 2016-06-06 | 2016-06-01 | 
    | 02979 | johny | notDoe | \null | 456766 | 02979 | 2016-06-06 19:00:32 | 1 | 0 | 2016-06-06 | 2016-06-01 | 
    | 02979 | johny | notDoe | \null | 457898 | 02979 | 2016-06-07 18:48:30 | 0 | 0 | 2016-06-07 | 2016-06-01 | 
    | 02979 | johny | notDoe | \null | 458947 | 02979 | 2016-06-08 06:59:45 | 1 | 0 | 2016-06-08 | 2016-06-01 | 

我尝试使用功能：

DATE_FORMAT(HourCard.date, "%Y-%m-%d") = DATE_FORMAT(HOUR.date,"%Y-%m-%d")
DATE(HourCard.date) = DATE(HOUR.date)
SUBSTRING(HourCard.date, 1, 10) = SUBSTRING(Hour.date, 1, 10)

但只插入如上所示的字符串会产生预期的结果 并且两个表中的编码是相同的。 Hour.date为date类型，HourCard.date为datetime。