MySQL左连接子查询null连接

Question

好的，所以我有一个相当详细的SQL查询，我有2个表，第一个表包含作业数据。 第二张表包含为该家庭作业设置的班级中每个学生的记录。

我想在满足某些条件的第二张表上统计一些数据，例如，向多少学生发放了家庭作业，计算了多少学生归还了作业，还计算了老师复习了多少作业。

我将在本文的底部发布我的查询。

我在2个左联接上使用子查询。 子查询如下所示：

LEFT JOIN (
    SELECT homework_id, COUNT(uid) AS total_returned
    FROM tbl_homework_student_log 
    WHERE homework_completed = 1
) r2 ON r2.homework_id = h.uid

在上面的示例中，学生日志表包含一个索引homework_id ，该索引链接回到主作业表的唯一索引h.uid 。

这种情况有效时发生我的问题：

1）如果COUNT(uid)等于0，则homework_id为NULL 。

2）查询产生以下结果

 r2.homework_id   |  total_returned (COUNT(uid))
------------------+------------------------------
 NULL             | 0

我的联接失败，并显示以下错误消息：

Column 'homework_id' cannot be null - (r2 ON r2.homework_id (null) = h.uid)

我不知道如何解决这个问题。 我最初在SELECT子句中有我的子查询，但是我想在LEFT JOINs中有它。

我的问题是如何确保r2.homework_id永远不会为null？ 我尝试了以下方法：

• 从联接子查询中删除了COUNT（）函数，并将其移至主选择子句-不能按预期工作
• 将每个联接子查询中的where子句移至与其关联的ON子句-不起作用
• 尝试在各自的ON子句中添加AND r2.homework_id IS NOT NULL不起作用
• 尝试在连接子查询的WHERE子句中添加AND COUNT(uid)>0不起作用

我全都没主意了。

这是我的整个查询：

SELECT h.uid, h.class_id, h.homework_details, h.require_upload_return,
       CONCAT(u.surname, ', ', u.forename) AS teacher_name, 
       DATE_FORMAT(h.set_date, '%D %M %Y') AS set_date_DMY,
       DATE_FORMAT(h.set_date, '%b %e, %Y') AS set_date_beY,
       UNIX_TIMESTAMP(h.set_date) AS set_date_timestamp,
       DATE_FORMAT(h.due_date, '%D %M %Y') AS due_date_DMY, 
       DATE_FORMAT(h.due_date, '%b %e, %Y') AS due_date_beY,
       UNIX_TIMESTAMP(h.due_date) AS due_date_timestamp, 
       IF(h.due_date<=DATE(NOW()), 1, 0) AS homework_due, 
       r1.total_issues, 
       IF(r2.total_returned IS NULL, 0, r2.total_returned) AS total_returned, 
       IF(h.due_date<=DATE(NOW()), r1.total_issues, IF(r2.total_returned IS NULL, 0, r2.total_returned)) AS waiting_review,
       h.resource_file
FROM tbl_homework h
INNER JOIN tbl_users u 
    ON u.uid = h.teacher_id
INNER JOIN (
    SELECT homework_id, COUNT(uid) AS total_issues 
    FROM tbl_homework_student_log
) r1 ON r1.homework_id = h.uid
LEFT JOIN (
    SELECT uid, homework_id, COUNT(uid) total_returned 
    FROM tbl_homework_student_log
    WHERE homework_completed = 1
) r2 ON r2.homework_id = h.uid 
LEFT JOIN (
    SELECT homework_id, COUNT(uid) waiting_review 
    FROM tbl_homework_student_log
    WHERE seen_by_issuer = 0
) r3 ON r3.homework_id = h.uid
WHERE h.teacher_id = ?
    AND h.set_date>=DATE_SUB(NOW(), INTERVAL 10 DAY) 
    AND h.homework_template = 0
ORDER BY h.class_id ASC, 
    h.set_date ASC

Answer 1

您实际上并不需要子查询，只需更少的三个子查询即可获取所需的数据。 我相信以下将解决问题：

SELECT h.uid,h.class_id, h.homework_details, h.require_upload_return,
       CONCAT(u.surname, ', ', u.forename) AS teacher_name, 
       DATE_FORMAT(h.set_date, '%D %M %Y') AS set_date_DMY,
       DATE_FORMAT(h.set_date, '%b %e, %Y') AS set_date_beY,
       UNIX_TIMESTAMP(h.set_date) AS set_date_timestamp,
       DATE_FORMAT(h.due_date, '%D %M %Y') AS due_date_DMY, 
       DATE_FORMAT(h.due_date, '%b %e, %Y') AS due_date_beY,
       UNIX_TIMESTAMP(h.due_date) AS due_date_timestamp, 
       IF(h.due_date<=DATE(NOW()), 1, 0) AS homework_due, 
       COUNT(*) AS total_issues, 
       SUM(IF(l.homework_completed, 1, 0)) AS total_returned,
       SUM(IF(l.seen_by_issuer = 0, 1, 0)) AS waiting_review,
       h.resource_file
FROM tbl_homework h
    INNER JOIN tbl_users u ON u.uid = h.teacher_id
    INNER JOIN tbl_homework_student_log l ON h.uid = l.homework_id
WHERE h.teacher_id = ?
    AND h.set_date>=DATE_SUB(NOW(), INTERVAL 10 DAY) 
    AND h.homework_template = 0
GROUP BY h.uid
ORDER BY h.class_id ASC, 
    h.set_date ASC

Answer 2

主要问题是您的子查询中缺少GROUP BY子句，因此您要对表中符合WHERE子句的所有内容进行计数，而不是按学生对它们进行计数。 同样，所有这些tbl_homework_student_log子查询都可以合并为一个查询。

SELECT h.uid, h.class_id, h.homework_details, h.require_upload_return,
       CONCAT(u.surname, ', ', u.forename) AS teacher_name, 
       DATE_FORMAT(h.set_date, '%D %M %Y') AS set_date_DMY,
       DATE_FORMAT(h.set_date, '%b %e, %Y') AS set_date_beY,
       UNIX_TIMESTAMP(h.set_date) AS set_date_timestamp,
       DATE_FORMAT(h.due_date, '%D %M %Y') AS due_date_DMY, 
       DATE_FORMAT(h.due_date, '%b %e, %Y') AS due_date_beY,
       UNIX_TIMESTAMP(h.due_date) AS due_date_timestamp, 
       IF(h.due_date<=DATE(NOW()), 1, 0) AS homework_due, 
       r.total_issues, 
       IFNULL(r.total_returned, 0) AS total_returned, 
       IF(h.due_date<=DATE(NOW()), r.total_issues, IFNULL(r.waiting_review, 0)) AS waiting_review,
       h.resource_file
FROM tbl_homework h
INNER JOIN tbl_users u 
    ON u.uid = h.teacher_id
INNER JOIN (
     SELECT homework_id, COUNT(*) AS total_issues
            SUM(homework_completed = 1) AS total_returned,
            SUM(seen_by_issuer = 0) AS waiting_review
     FROM tbl_homework_student_log
     GROUP BY homework_id
) r ON r.homework_id = h.uid
WHERE h.teacher_id = ?
    AND h.set_date>=DATE_SUB(NOW(), INTERVAL 10 DAY) 
    AND h.homework_template = 0
ORDER BY h.class_id ASC, 
    h.set_date ASC