如何查询列中具有特定值的最新记录

Question

我有一个学校数据库（只有 3 个）。在数据库中，提交有关其性能的记录。

我想要一个选项让用户只查询每所学校的最新条目。

目前 - 我可以查询最近的整体...

$result = mysqli_query($con,"SELECT * FROM reports, academicyears,schools,terms
WHERE reports.report_id=(SELECT MAX(report_id) FROM reports WHERE school_id=1 OR school_id=2 OR school_id=3)
AND academicyears.ay_id=reports.ay_id
AND schools.school_id=reports.school_id
AND terms.term_id=reports.term_id
ORDER BY reports.datesubmitted DESC

但我希望能够同时找到学校 1、学校 2 和学校 3 的最新信息。

我可以看到 OR 没有给我我想要的……它只是返回最新的记录！查询应该是什么？

另外 - 下一步是查找最近的记录，而无需命名学校 ID。随着学校数量的增长——那将是完全低效的！这可能吗？

Answer 1

感谢所有帮助。提供的解决方案之一建议使用 UNION 查询。这导致了“重复列”错误。所以，我重命名了数据库中的列并更新了查询语言，它现在可以工作了！

$result = mysqli_query($con,"SELECT * FROM (
    (
        SELECT *
        FROM reports, academicyears, schools, terms 
        WHERE reports.report_id=(
            SELECT MAX(report_id) 
            FROM reports 
            WHERE rep_school_id=1
        )
        AND academicyears.ay_id=reports.rep_ay_id
        AND schools.school_id=reports.rep_school_id
        AND terms.term_id=reports.rep_term_id
    ) UNION (
        SELECT *
        FROM reports, academicyears, schools, terms 
        WHERE reports.report_id=(
            SELECT MAX(report_id) 
            FROM reports 
            WHERE rep_school_id=2
        )
        AND academicyears.ay_id=reports.rep_ay_id
        AND schools.school_id=reports.rep_school_id
        AND terms.term_id=reports.rep_term_id
    ) UNION (
        SELECT *
        FROM reports, academicyears, schools, terms 
        WHERE reports.report_id=(
            SELECT MAX(report_id) 
            FROM reports 
            WHERE rep_school_id=3
        )
        AND academicyears.ay_id=reports.rep_ay_id
        AND schools.school_id=reports.rep_school_id
        AND terms.term_id=reports.rep_term_id
    )
) AS Latest
ORDER BY datesubmitted DESC
");?>

再次感谢任何提供帮助的人。

Answer 2

您正在寻找不在 mysql 中但可以模拟如下的窗口函数：

SELECT
  *
FROM
  reports, academicyears,schools,terms   ,
  (
    SELECT 
      GROUP_CONCAT(top_codes_per_group) AS top_codes
    FROM
     (
       SELECT 
      SUBSTRING_INDEX(GROUP_CONCAT(report_id.   ORDER By datesubmitted DESC), ',', 5) AS top_codes_per_group
    FROM
      reports, academicyears,schools,terms       
      WHERE school_id=1 OR school_id=2 OR.   school_id=3)
AND academicyears.ay_id=reports.ay_id
AND schools.school_id=reports.school_id
AND terms.term_id=reports.term_id GROUP BY

  ) s_top_codes_per_group
  ) s_top_codes
WHERE
  FIND_IN_SET(Code, top_codes)
ORDER BY
 School,
 Datesubmitted DESC
;

这给出了每所学校排名前 5 位的报告。

Answer 3

这样的查询应该可以解决问题。这是三个单独的查询，每个学校 1 个，由 UNION 命令组合在一起：

FROM (
    (
        SELECT * 
        FROM reports, academicyears, schools, terms
        WHERE reports.report_id=(
            SELECT MAX(report_id) 
            FROM reports 
            WHERE school_id=1
        )
        AND academicyears.ay_id=reports.ay_id
        AND schools.school_id=reports.school_id
        AND terms.term_id=reports.term_id
    ) UNION (
        SELECT * 
        FROM reports, academicyears, schools, terms
        WHERE reports.report_id=(
            SELECT MAX(report_id) 
            FROM reports 
            WHERE school_id=2
        )
        AND academicyears.ay_id=reports.ay_id
        AND schools.school_id=reports.school_id
        AND terms.term_id=reports.term_id
    ) UNION (
        SELECT * 
        FROM reports, academicyears, schools, terms
        WHERE reports.report_id=(
            SELECT MAX(report_id) 
            FROM reports 
            WHERE school_id=3
        )
        AND academicyears.ay_id=reports.ay_id
        AND schools.school_id=reports.school_id
        AND terms.term_id=reports.term_id
    )
) AS Latest
ORDER BY datesubmitted DESC

从技术上讲，这是在 1 中运行 6 个查询，因此效率不会很高。随着条目数变大，这个查询会变慢。