php - mysql join重复行，order by引起的问题

Question

我正在尝试获取每个类(class)的名称、该类(class)的学生以及分配给该类(class)的科目。

这里，问题是由 ORDER BY 引起的，我已将 ORDER BY 分配给 student.id。

这会导致主题行重复。

如果未分配 ORDER BY，则按预期提取主题行，且不重复，但学生行重复。

表格结构

学生

id  |   Rid |   class   |   name
================================
1   |   1   |   1       |   John
2   |   2   |   1       |   Harsita

类别

id  |   title
==============
1   |   One
2   |   two
3   |   three

主题

id  |   title
===============
1   |   science
2   |   math
3   |   english
4   |   social
5   |   hpe

类(class)主题

id  |   class   |   subject
===========================
1   |   1       |   1   
2   |   1       |   2
3   |   1       |   3
4   |   1       |   4

要求是

class: One
Science | Math | English | Social | Hpe | 
John | Harsita 

但是我得到了什么

如果将 ORDER BY 分配给 student.id

class: One
Science | Math | English | Social | Hpe | Math | English | Social | Hpe | Science | 
John | Harsita |

如果ORDER BY尚未分配给student.id

class: One
Science | Math | English | Social | Hpe | 
John | Harsita | John | Harsita | John | Harsita | John | Harsita | John | Harsita | 

我尝试使用GROUP BY到subject.id，它只显示一个学生(第一行:John)。问题出在哪里？如何在不使用 subquery 或 GROUP_CONCAT

的情况下解决问题

代码 - php 和 mysql(使用 ORDER BY)

//mysql query
    SELECT 
        DISTINCT class_subject.class, 
        subject.title AS sub,
        student.name AS stdt,
        class.title AS class
    FROM 
        student
    INNER JOIN class_subject ON class_subject.class=student.class
    INNER JOIN subject ON subject.id=class_subject.subject
    INNER JOIN class ON class.id=student.class
    WHERE
        student.class=:cid;

//php
    $class='';
    $stdt='';
    $sub='';
    $results = Array();
        while($row=$result->fetch(PDO::FETCH_ASSOC)){
            if ($row['class'] != $class){
                $class = $row['class'];
                echo "Class: ".$class."<br/>";
            }
            if ($row['sub'] != $sub){
                $sub = $row['sub'];
                echo $sub." | ";
            }
            $results[]=$row;
        }
        echo "<br/>";
        foreach ($results as $row) {
            if ($row['stdt'] != $stdt) {
                $stdt = $row['stdt'];
                echo $stdt." | ";
            }
        }

Answer 1

查看您的预期结果样本，您似乎需要名称和主题的聚合结果

SELECT 
    group_concat( subject.title) AS sub,
    group_concat(student.name) AS stdt,
    class.title AS class
FROM 
    student
INNER JOIN class_subject ON class_subject.class=student.class
INNER JOIN subject ON subject.id=class_subject.subject
INNER JOIN class ON class.id=student.class
WHERE student.class=:cid
group by class.title

使用聚合函数和组，您可以在同一行上显示聚合结果