Sql连接两个不同的表

Question

There are colums in tables:

friends:  id | id1 | id2 | accepted | notification

users:    id | username | email | password

id1 in friends is always sender of friends request and id2 is his friend.

$sql = "SELECT * FROM friends WHERE id1='$id' AND accepted=1 AND notification=1;";
$result = mysqli_query($conn, $sql);
$data = mysqli_fetch_all($result, MYSQLI_ASSOC);

header("Content-Type: application/json");
echo json_encode($data);

$id is taken from session. I want to get username from users where id(in users)=id2(if friends)

how will $sql look like?

Answer 1

Not sure what you are trying to achieve in logic. What does this means : id(in users)=id2(if friends)? can you describe in words

Maybe something like this might give an insight on how to proceed :

SELECT 
   sender.id1 as senderID,
   u.username as senderUsername,
   receiver.id1 as receiverID,
   u.username as receiverUsername
FROM users u 
INNER JOIN friends sender ON u.id = sender.id1
INNER JOIN friends receiver ON u.id = receiver.id2
WHERE sender.id1='$id' 
  AND receiver.accepted=1
  AND receiver.notification=1;

Answer 2

Use this query to get ypur desired results.

$sql = "select f.id, f.id1,f.id2 , f.accepted,f.notification,u.id, u.username,u.email, u.password from friends as f JOIN users as u ON u.id = f.id2 Where f.accepted = '1' AND f.notification = '1' ";