Question

有没有办法在最后一个子查询中包含IF语句（或者在其他地方，如果更合适的话），以排除 class_id IF 上计数的行在同一张表的考勤栏中对应 0 ？

我想要做的是，如果一个6人已经预订了一个班级（最大大小6），但是1个班级已经取消了 - 我希望结果为5，从而显示班级可用（为什么我依靠班级ID而非考勤领域）

SELECT c.*
FROM classes c
WHERE c.location_id = :location_id
AND (c.level_id = :current_level OR c.level_id = :previous_level OR c.level_id = :next_level)
AND c.datetime BETWEEN CURDATE() AND ADDDATE(CURDATE(),INTERVAL 14 DAY)
AND (c.class_id NOT IN (
     SELECT class_id 
     FROM swimmer_classes) 
     OR c.class_id IN (
                       SELECT class_id 
                       FROM swimmer_classes 
                       GROUP BY class_id 
                       HAVING COUNT(class_id) < 6))"

更新1

swimmer_classes schema

swimmer_class_id   swimmer_id    class_id   attendance
1                  1             1          null           
2                  2             1          null
3                  3             1          null
4                  4             1          null
5                  5             1          null
6                  6             1          0

结果：Count（class_id）= 6

所需输出

根据以下伪代码

计算所需数= 5

 Count(class_id) - Count(attendance of class_id) = 5

Answer 1

这样的事情会在你的场景中起作用吗？

SELECT class_id 
FROM swimmer_classes
WHERE attendance <> 0 OR attendance is NULL
GROUP BY class_id 
HAVING COUNT(class_id) < 6

或者更啰嗦：

SELECT class_id FROM 
(
    SELECT class_id, count(*) as cnt
    FROM swimmer_classes
    WHERE attendance <> 0 OR attendance is NULL
    GROUP BY class_id 
) t
where t.cnt < 6

如何从另一个计数结果中减去计数结果以满足mysql中的where子句？

1 个答案: