PHP / MySQL-好友列表SQL查询

Question

I'm going based off this guide in a previous StackOverflow post:

"Table Name: User
Columns:
    UserID PK
    EmailAddress
    Password
    Gender
    DOB
    Location

TableName: Friends
Columns:
    UserID PK FK
    FriendID PK FK
    (This table features a composite primary key made up of the two foreign 
     keys, both pointing back to the user table. One ID will point to the
     logged in user, the other ID will point to the individual friend
     of that user)

Example Usage:

Table User
--------------
UserID EmailAddress Password Gender DOB      Location
------------------------------------------------------
1      bob@bob.com  bobbie   M      1/1/2009 New York City
2      jon@jon.com  jonathan M      2/2/2008 Los Angeles
3      joe@joe.com  joseph   M      1/2/2007 Pittsburgh

Table Friends
---------------
UserID FriendID
----------------
1      2
1      3
2      3"

Now, I have all that done, and I've created this query:

if ($_SESSION["user_id"]) {
    $user_id = $_SESSION["user_id"];
    $query = "SELECT * ";
    $query .= "FROM friends ";
    $query .= "WHERE ";
    $query .= "user_id OR friend_id = '{$user_id}' ";
    $result = mysqli_query($connection, $query);
    $result_set = mysqli_fetch_assoc($result);
    print_r($result_set);

Yay! This print_r gets the associated array that I expected. But now, because either user_id OR friend_id can be the logged in user, I'm stumped s to what kind of query I need to go with to be able to actually display the friends list.

$query = "SELECT * ";
$query .= "FROM users ";
$query .= "WHERE id = ''

This is about as far as I've gotten due to my confusion. Any point in the right direction would be amazing. thank you!

Answer 1

PHP

$reg_no = $_SESSION["user_id"];

SQL QUERY:

select * from `User` join `friends` on '$reg_no' = `friends`.`UserID` and `User`.`UserID` = `friends`.`UserID` or '$reg_no' = `FriendID` and `User`.`UserID` = `friends`.`UserID`;

TRY IT ....

Answer 2

Oh I get it. You (the user) want to display your friends (other users)

Try this:

SELECT U.EmailAddress, U.Gender, U.DOB, U.Location FROM User U
LEFT JOIN Friends F on U.UserID = F.Friend_ID WHERE F.User_ID = 1;

And then just change 1 to whatever the logged in user's id is.

Cheers!