Php 如何使用SQL内部联接&；使用外键从数据库获取数据的别名？

我试图在表中显示数据，如下代码所示。我试图从名为用户的数据库表中获取请购单表（视图）中这3列的数据：

征用

批准人和

支付人

数据库中的表格如下所示。我将省略那些似乎工作正常的列

财务申请

| req_id | req_by | approved_by | disbursed_by |
| ------ | ------ | ----------- | ------------ |
|    1    |    1    |     2     |      3       |
|    2    |    1    |     2     |      3       |

用户表格

| id | username |
| ------ | ------ | 
|    1    |    Lisa    | 
|    2    |    Anne    |
|    3    |    John    |

即使应该是不同的用户名，输出也始终为Lisa。我是一个noob，但从我所学到的到目前为止，这个问题可以通过正确使用SQL别名轻松解决，但我不知道如何解决，所以我问，我该如何修复这个专家

 <table class="table table-striped">
                <thead>
                    <tr>
                        <td>ID</td>
                        <td>File</td>
                        <td>Expense Category</td>
                        <td>Amount Requisitioned</td>
                        <td>Details</td>
                        <td>Requisitioned By</td>
                        <td>Requisition Date</td>
                        <td>Approval Status</td>
                        <td>Amount Approved</td>
                        <td>Approved By</td>
                        <td>Date Approved</td>
                        <td>Disbursement Status</td>
                        <td>Amount Disbursed</td>
                        <td>Disbursed By</td>
                        <td>Date Disbursed</td>
                        <td>Note</td>
                        <td colspan=2>Actions</td>
                    </tr>
                </thead>
                <tbody>
                    <hr>

                    <?php
                    $sql = "
        SELECT fin_requisition.req_id            
             , ops_files.file_name
             , fin_expense_cats.expense_cat
             , fin_requisition.amount
             , fin_requisition.details
             , users.username
             , fin_requisition.req_date
             , fin_approval_status.status
             , fin_requisition.amt_approved
             , users2.username
             , fin_requisition.approval_date
             , fin_disb_status.status
             , fin_requisition.amt_disbursed
             , users3.username
             , fin_requisition.date_disbursed
             , fin_requisition.notes FROM fin_requisition
          JOIN ops_files 
            ON fin_requisition.file = ops_files.file_id
          JOIN fin_expense_cats 
            ON fin_requisition.expense_cat = fin_expense_cats.cat_id
          JOIN users 
            ON fin_requisition.req_by = users.id 
          JOIN fin_approval_status 
            ON fin_requisition.approval_status = fin_approval_status.status_id
          JOIN users users2 
            ON fin_requisition.approved_by = users2.id 
          JOIN fin_disb_status 
            ON fin_requisition.disb_status = fin_disb_status.status_id 
          JOIN users users3 
            ON fin_requisition.disbursed_by = users3.id 
         ORDER 
            BY req_id ASC
            ";
                    $result = $mysqli->query($sql);
                    ?>

                    <?php foreach ($result as $row) : ?>
                        <tr>
                            <td><?php echo $row["req_id"]; ?></td>
                            <td><?php echo $row["file_name"]; ?></td>
                            <td><?php echo $row["expense_cat"]; ?></td>
                            <td><?php echo $row["amount"]; ?></td>
                            <td><?php echo $row["details"]; ?></td>
                            <td><?php echo $row["username"]; ?></td>
                            <td><?php echo $row["req_date"]; ?></td>
                            <td><?php echo $row["status"]; ?></td>
                            <td><?php echo $row["amt_approved"]; ?></td>
                            <td><?php echo $row["username2"]; ?></td>
                            <td><?php echo $row["approval_date"]; ?></td>
                            <td><?php echo $row["status"]; ?></td>
                            <td><?php echo $row["amt_disbursed"]; ?></td>
                            <td><?php echo $row["username3"]; ?></td>
                            <td><?php echo $row["date_disbursed"]; ?></td>
                            <td><?php echo $row["notes"]; ?></td>
                            <td>
                                <a href="edit_requisition.php?req_id=<?php echo ($row["req_id"]); ?>" class="btn btn-primary">Edit</a>
                            </td>
                            <td>
                                <a href="delete_requisition.php?req_id=<?php echo ($row["req_id"]); ?>" method="post">
                                    <button class="btn btn-danger" type="submit" onclick="return confirm('Are you sure you want to delete this record?')">Delete</button>
                                </a>
                            </td>
                        </tr>
                    <?php endforeach; ?>
                </tbody>
            </table>

身份证件
文件
费用类别
征用金额
细节
征用
申请日期
批准状态
核准金额
批准人
批准日期
支付状况
支付金额
支付人
支付日期
注
行动

在sql中，您应该为不同的用户名使用prpoer别名，例如：username usename2，username3

       SELECT fin_requisition.req_id            
         , ops_files.file_name
         , fin_expense_cats.expense_cat
         , fin_requisition.amount
         , fin_requisition.details
         , users.username 
         , fin_requisition.req_date
         , fin_approval_status.status
         , fin_requisition.amt_approved
         , users2.username username2
         , fin_requisition.approval_date
         , fin_disb_status.status
         , fin_requisition.amt_disbursed
         , users3.username username3
         , fin_requisition.date_disbursed
         , fin_requisition.notes FROM fin_requisition
      JOIN ops_files 
        ON fin_requisition.file = ops_files.file_id
      JOIN fin_expense_cats 
        ON fin_requisition.expense_cat = fin_expense_cats.cat_id
      JOIN users 
        ON fin_requisition.req_by = users.id 
      JOIN fin_approval_status 
        ON fin_requisition.approval_status = fin_approval_status.status_id
      JOIN users users2 
        ON fin_requisition.approved_by = users2.id 
      JOIN fin_disb_status 
        ON fin_requisition.disb_status = fin_disb_status.status_id 
      JOIN users users3 
        ON fin_requisition.disbursed_by = users3.id 
     ORDER 
        BY req_id ASC

然后在php代码中引用这些列别名

<td><?php echo $row["req_id"]; ?></td>
<td><?php echo $row["file_name"]; ?></td>
<td><?php echo $row["expense_cat"]; ?></td>
<td><?php echo $row["amount"]; ?></td>
<td><?php echo $row["details"]; ?></td>
<td><?php echo $row["username"]; ?></td>
<td><?php echo $row["req_date"]; ?></td>
<td><?php echo $row["status"]; ?></td>
<td><?php echo $row["amt_approved"]; ?></td>
<td><?php echo $row["username2"]; ?></td>
<td><?php echo $row["approval_date"]; ?></td>
<td><?php echo $row["status"]; ?></td>
<td><?php echo $row["amt_disbursed"]; ?></td>
<td><?php echo $row["username3"]; ?></td>
<td><?php echo $row["date_disbursed"]; ?></td>
<td><?php echo $row["notes"]; ?></td>

---

这里有很多问题

让我们从更简单的事情开始

考虑以下几点：

DROP TABLE IF EXISTS requisitions;

CREATE TABLE requisitions
(req_id SERIAL PRIMARY KEY
,req_by INT NOT NULL
,approved_by INT NOT NULL
,disbursed_by INT NOT NULL
);

INSERT INTO requisitions VALUES
(1,2,4,3),
(2,4,4,2);

DROP TABLE IF EXISTS users ;

CREATE TABLE users 
(id SERIAL PRIMARY KEY
,username VARCHAR(12) UNIQUE
);

INSERT INTO users VALUES
(1,'Lisa'),
(2,'Anne'),
(3,'John');

SELECT x.req_id 
     , r.username AS requisitioner -- the AS keyword is optional
     , a.username AS approver      -- and often
     , d.username AS disburser     -- omitted
  FROM requisitions x 
  LEFT 
  JOIN users r 
    ON r.id = x.req_by 
  LEFT 
  JOIN users a 
    ON a.id = x.approved_by 
  LEFT 
  JOIN users d
    ON d.id = x.disbursed_by;
+--------+---------------+----------+-----------+
| req_id | requisitioner | approver | disburser |
+--------+---------------+----------+-----------+
|      1 | Anne          | NULL     | John      |
|      2 | NULL          | NULL     | Anne      |
+--------+---------------+----------+-----------+

---

所以，没有Lisa。

似乎，对于您提供的示例数据，您需要将JOIN更改为lEFT JOIN。请看下面

 WITH REQ(req_id,req_by,approved_by,disbursed_by) AS
  (
     SELECT 1,2,4,3
       UNION ALL
     SELECT 2,4,4,2
  ),
 USERS(ID,USERNAME) AS
  (
     SELECT   1 ,  'Lisa'  
        UNION ALL
     SELECT 2,'Anne'
        UNION ALL
     SELECT 3,'JOHN'

  )
   SELECT R.REQ_ID,R.req_by,COALESCE(R_BY.USERNAME,'') AS REQUESTER,
    R.approved_by,COALESCE(APPRR_BY.USERNAME,'')AS APPROVER,
    R.disbursed_by,COALESCE(DISB_BY.USERNAME,'')AS DISBURSER
    FROM REQ AS R
     LEFT JOIN USERS AS R_BY ON R.req_by=R_BY.ID
     LEFT JOIN USERS AS APPRR_BY ON R.req_by=APPRR_BY.ID
     LEFT JOIN USERS AS DISB_BY ON R.disbursed_by=DISB_BY.ID

---

@草莓nvm看起来应该这样。他们加入没问题，你应该在选择列表中使用

users2.username作为username2

，与users3相同。否则他们会得到生成的名称，然后是'username'@Turo并修改php foreach代码？当然，在php代码中使用username2和username3，如果你是对的，我认为示例数据被破坏了，否则根本不会有结果。谢谢你的回答。我已经对它进行了改进，这样它就不再误导用户了。不过，您仍然没有使用列别名。