How do I use SQL INNER JOIN & ALIAS with foreign keys to fetch data from a database?

Question

I am trying to display data in a table as the code below shows. I'm trying to get data for these 3 columns in Requisitions table(view) from a database table called users :

1. Requisitioned By
2. Approved By and
3. Disbursed By

Tables in the db are as follows. I am going to omit columns that seem to be working correctly.

fin_requisitions

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

users table

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

The output is always Lisa even when it should be a different username. I'm a noob but from what I've learnt so far the problem can be easily solved by using SQL ALIAS correctly but I have no idea how so I ask, how do I fix this gurus?

 <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>

Answer 1

In sql you should use prpoer alias name for the diffent username eg: 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

then refer to these column alias in php code

<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>

Answer 2

There's an awful lot of question here.

Let's start with something simpler...

Consider the following:

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      |
+--------+---------------+----------+-----------+

So, no Lisa.

Answer 3

Seems, for sample data you have provided you need to change JOIN to lEFT JOIN. Please see below

 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