从一个数据库中的多个表中选择最新记录（MYSQL PHP）

Question

我有学校 mysql 数据库，有 15 个表（15 个教室），每个表都有统一的字段/简单数据，例如：ID、DATE、TIME、CLASSROOM_NO、STUDENTS_ALL、STUDENTS_CURRENT、COMMENTS

方法是能够通过增加该数据库的php表单一直更新这些数据，这很好，它工作正常。

在另一个 php 页面中，我有 display.php，它只显示添加的最新记录，所以我可以知道这个教室的最新更新，这也很好，正是我需要的，这是我的代码：

<?php
$con=mysqli_connect("localhost","classroom","mypsassword","mysqluser");
// Check connection
if (mysqli_connect_errno())
  {
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
  }

$result = mysqli_query($con,"SELECT id, date, time, classroom_no, students_all,     student_current,comments FROM classroom_1 ORDER BY id DESC LIMIT 1");


while($row = mysqli_fetch_array($result))
 {

 echo "<b>Room:</b> " . $row['classroom_no'] . "<br>";
 echo "<b>All students:</b> " . $row['students_all'] . "<br>";
 echo "<b>Available students:</b> " . $row['students_current'] . "<br>";
 echo "<b>Absent:</b> ";
 echo floatval($row['students_all'])  -  floatval($row['students_current']);
 echo "<br>";
 echo "<b>Updated on:</b> " . $row['date'] ."<br>";
 echo "<b>Update time:</b> " . $row['time']."<br>" ;

  }

mysqli_close($con);

?>

现在，我在一个页面中实现和检索每个教室的最新更新没有问题......我努力的是从每个表中选择最新记录，然后在同一页面中相应地显示它们。 . 所以要有一个php页面说all_classrooms.php

并在一页中显示所有教室（仅每个表的最新记录更新），因此我无法理解如何同时从所有表中选择 sql 查询并将其限制为最新记录...

我的表按如下顺序排列： 教室_1 教室_2 classrrom_3

请指教

Answer 1

最好重新设计您的数据库，用一张表用于教室，另一张用于班级（每个教室有很多行）等。

但是，如果这不可能，您将需要这样的东西：-

SELECT classroom, id, date, time, classroom_no, students_all, student_current, comments
FROM
(
    SELECT '1' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_1
    UNION
    SELECT '2' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_2
    UNION
    SELECT '3' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_3
    UNION
    SELECT '4' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_4
    UNION
    SELECT '5' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_5
    UNION
    SELECT '6' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_6
    UNION
    SELECT '7' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_7
    UNION
    SELECT '8' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_8
    UNION
    SELECT '9' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_9
    UNION
    SELECT '10' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_10
    UNION
    SELECT '11' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_11
    UNION
    SELECT '12' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_12
    UNION
    SELECT '13' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_13
    UNION
    SELECT '14' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_14
    UNION
    SELECT '15' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_15
) Sub1
INNER JOIN
(
    SELECT classroom, MAX(id) AS maxid
    FROM
    (
        SELECT '1' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_1
        UNION
        SELECT '2' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_2
        UNION
        SELECT '3' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_3
        UNION
        SELECT '4' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_4
        UNION
        SELECT '5' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_5
        UNION
        SELECT '6' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_6
        UNION
        SELECT '7' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_7
        UNION
        SELECT '8' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_8
        UNION
        SELECT '9' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_9
        UNION
        SELECT '10' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_10
        UNION
        SELECT '11' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_11
        UNION
        SELECT '12' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_12
        UNION
        SELECT '13' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_13
        UNION
        SELECT '14' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_14
        UNION
        SELECT '15' AS classroom, id, date, time, classroom_no, students_all, student_current, comments FROM classroom_15
    ) Sub3
    GROUP BY classroom
) Sub2
ON Sub1.classroom = Sub2.classroom AND Sub1.id = Sub2.maxid

请注意，我在 UNION 中添加了一个固定的教室栏。这可能会重复classic_no 的值。

还有一个名为students_all的列。这表明您有一个包含学生列表的列。如果是这样，最好将其拆分到另一个表中，该表可以为每个班级有很多行（班级中的每个学生一个）

Answer 2

正如 Kickstart 所建议的：我建议您规范化您的数据库设计。将所有教室数据放入一个带有列的表格中，以显示任何特定行与哪个教室相关。

我不推荐的其他（解决方法）解决方案是create a view。看起来像

CREATE VIEW allData AS
SELECT * from Classroom1
UNION ALL SELECT * from Classroom2
....

然后从此视图中编写选择。