SQL / PHP-根据列值两次调用同一表

Question

Here is a general setup of my table:

users
-------------
user_id | user_first | user_last | user_friendlist
1       | John       | Doe       | 2,3
2       | Jimmy      | John      | 1,3
3       | Papa       | John      | 1,2

1. First, I will select the above table using user_id and retrieve user_friendlist.
2. Then, I want to use user_friendlist and retrieve user_first & user_last.

I tried Left JOIN, but I am not sure how to call the same table twice... Please help! It gets all confusing when attempting to do a prepare statement.

General code for the first part:

$selectData = $con->prepare("SELECT 
                                    user_friendlist
                                FROM 
                                    users
                                WHERE 
                                    user_id=?");


    $selectData->bind_param("i", $user_id); 

    $user_id = $_POST['user_id'];

    $selectData -> execute(); 
    $selectData-> bind_result($user_friendlist);    

    $arr = array();
    while (  $selectData -> fetch() ) {
        $arr[] = $user_friendlist;
    }

    echo json_encode($arr);

Answer 1

you should have two tables users and friends

users will be like

| user_id | user_first | user_last | 
| 1       | John       | Doe       |
| 2       | Jimmy      | John      |
| 3       | Papa       | John      |

and friends

| user_id | freind_id | 
| 1       | 2         |
| 1       | 3         |
| 2       | 1         |
| 2       | 3         |
| 3       | 1         |
| 3       | 2         |

user_id is the program key for table users

user_id and friend_id are the primary key for table friend

friends.user_id and friends.friend_id is foreign keys for users.user_id

the query will be like

SELECT users2.user_first AS friend_first,users2.user_last AS friend_last
-- ,users1.user_first AS user_first,users1.user_last AS user_last 
-- uncomment the last line to get the user info with his friends
FROM users as users1
join friends
ON users1.user_id=friends.user_id
JOIN users AS users2
ON users2.user_id=friends.friend_id
WHERE users1.user_id=1

the query result is

| friend_first | friend_last |
| Jimmy        | John        |
| Papa         | John        |