使用php和mysql从表连接中获取数据

Question

我的数据库中有以下表格：

CREATE TABLE subjects (
  subject_id int(11) NOT NULL AUTO_INCREMENT,
  subject text,
  PRIMARY KEY (subject_id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 AUTO_INCREMENT=1;

CREATE TABLE users (
  id int(11) NOT NULL AUTO_INCREMENT PRIMARY KEY,
  username varchar(100) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

CREATE TABLE users_subjects (
  users_subjects_id int(11) NOT NULL AUTO_INCREMENT,
  user_id_fk int(11),
  subject_id_fk int(11),
  FOREIGN KEY(user_id_fk) REFERENCES users(id),
  FOREIGN KEY(subject_id_fk) REFERENCES subjects(subject_id),
  PRIMARY KEY (users_subjects_id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 AUTO_INCREMENT=1;

在表“ users_subjects”中，我将表“ subjects”和“ users”相关联。

所有表数据都是通过PHP引入的，当引入user和subjects时，会创建'users_subjects'表。

例如，如果我输入一个ID为1的名为“ EXAMPLE”的用户，ID为1和2的SAT和ICT主题，则“ users_subjects”表将类似于以下内容：

问题是我试图在PHP表中向用户显示他们的相应主题，但是我不知道如何在“主题”列中显示以逗号分隔的主题。我想做类似下图的操作。有人能帮我吗？谢谢

这就是我的代码中的内容：

 <table class="users">
    <tr>
        <th colspan="2"><h2>USERS</h2></th>
    </tr>
    <tr>
        <th> NAME </th>
        <th> SUBJECTS </th>
    </tr>

<?php
$sql = "SELECT * FROM users WHERE rol='profesor'"; 
$result = $conn->query($sql); if ($result->num_rows==0){

    echo 'No users';
}else{
while($row = $result->fetch_assoc()) {


     $subjects = "";
        $sql2 = "SELECT user_id_fk, GROUP_CONCAT(subject_id_fk SEPARATOR ',') FROM users_subjects WHERE user_id_fk='$id' GROUP BY user_id_fk;";

        /*$sql2 = "SELECT user_id_fk, GROUP_CONCAT(subject_id_fk SEPARATOR ',') FROM users_subjects where user_id_fk='$id'";*/


        $result2 = $conn->query($sql2);
        while ($row2 = $result2->fetch_assoc()) {
            $subjects = $row2["subject"];
        }

        echo "<tr>
                  <td>".$row["username"]."</td>
                  <td>".$subjects."</td></tr>";
    }
}
?>
</table>

Answer 1

您需要的查询必须包含分组才能使用GROUP_CONCAT：

SELECT user_id_fk, GROUP_CONCAT(subject_id_fk SEPARATOR ',')
  FROM users_subjects
  WHERE user_id_fk=?
  GROUP BY user_id_fk

如果要获取名称，则需要JOIN：

SELECT users.username, GROUP_CONCAT(subjects.subject SEPARATOR ',')
  FROM users_subjects
  RIGHT JOIN users ON users.id=users_subjects.user_id_fk
  RIGHT JOIN subjects ON subjects.subject_id=users_subjects.subject_id_fk
  WHERE user_id_fk=?
  GROUP BY user_id_fk