Question

一直撞到墙上，无法解决这个问题：\

SELECT
  `people`.*,
  `students`.*,
  `student_class_relationships`.*,
  `geo_checkin_on_campus`.`datetime_created` as checkin_time
FROM `student_class_relationships`
  LEFT OUTER JOIN `students`
    ON `student_class_relationships`.`student` = `students`.`id`
  LEFT OUTER JOIN `people`
    ON `students`.`student` = `people`.`id`
  LEFT OUTER JOIN `geo_checkin_on_campus`
    ON `students`.`id` = (
      SELECT MIN(`geo_checkin_on_campus`.`student`)
      FROM `geo_checkin_on_campus`
      WHERE `geo_checkin_on_campus`.`student` = `students`.`id`
    )
 WHERE `class` = 56

预期结果是许多行，每个students.id只有一个条目。

这是我的schema

Answer 1

从性能角度来看，这不是最好的查询，

但只是为了解决您的问题，这是我的尝试：

SELECT
  `people`.*,
  `students`.*,
  `student_class_relationships`.*,
  geoCheckinOnCampus.datetimeCreated as checkin_time
FROM `student_class_relationships`
  LEFT JOIN `students`
    ON `student_class_relationships`.`student` = `students`.`id`
  LEFT JOIN `people`
    ON `students`.`student` = `people`.`id`
  LEFT JOIN 
    (
      SELECT 
        student,
        MAX(datetime_created) datetimeCreated
      FROM `geo_checkin_on_campus`
      GROUP BY `student`
    ) geoCheckinOnCampus
  ON `students`.`id` = geoCheckinOnCampus.`student`
 WHERE `class` = 56

注意根据@xQbert的回答，如果您正在寻找最新的日期时间，我会将MIN更改为MAX函数。

Answer 2

如果我假设你想要go_checkin_on_Campus中每个学生的最新签到（而不是最早创建的日期）那么......

SELECT
  `people`.*,
  `students`.*,
  `student_class_relationships`.*,
  B.`datetime_Updated` as checkin_time
FROM `student_class_relationships`
  LEFT OUTER JOIN `students`
    ON `student_class_relationships`.`student` = `students`.`id`
  LEFT OUTER JOIN `people`
    ON `students`.`student` = `people`.`id`
  LEFT OUTER JOIN (
      SELECT max(datetime_updated), student
      FROM `geo_checkin_on_campus`
      group by student
) B
    ON `students`.`id` = B.Student
 WHERE `class` = 56

Answer 3

注意：这是一个可能的答案。我将根据OP的评论编辑/修改它。

这基本上什么也没做。这和加入学生ID一样好

LEFT OUTER JOIN `geo_checkin_on_campus`
    ON `students`.`id` = (
      SELECT MIN(`geo_checkin_on_campus`.`student`)
      FROM `geo_checkin_on_campus`
      WHERE `geo_checkin_on_campus`.`student` = `students`.`id`
    )

如果您希望min（或最早）datetime_created使用类似

的内容

LEFT OUTER JOIN (
      SELECT `geo_checkin_on_campus`.`student` student,
              MIN(`geo_checkin_on_campus`.`datetime_created`) dt
      FROM   `geo_checkin_on_campus`
      WHERE  `geo_checkin_on_campus`.`student` = `students`.`id`
      GROUP BY `geo_checkin_on_campus`.`student`
    ) t
    ON `students`.`id` = t.student

LEFT JOIN只是多对一的最新一行

3 个答案: