MySQL INNER JOIN using an OR whilst logging in - php

Question

My table structure is as follows:
~ MEMBERS ~
UID (a-i)
NAME
EMAIL
~ COUPLES ~
UID_1
UID_2
PASSWORD
SALT
When a couple sign up, their UIDs get inserted in to the couples table, along with their joint password (and its salt). Now, when logging in, I need to join the tables so I can check the password for either user.
i.e. Find Email Address > See which Couple they are in > Check against Password
This is my current query:
$query = "SELECT * FROM MEMBERS m
INNER JOIN COUPLES c ON ((c.UID_1 = m.UID) OR (c.UID_2 = m.UID))
WHERE EMAIL = '$email'";
(I'm only using * for now as I can't figure out exactly what I need to select for the INNER JOIN to work.)
And the rest of the code on login-script.php is as follows:
$result = mysqli_query($con, $query);
if(mysqli_num_rows($result) == 0) {
header('Location: index.php?msg=error4');
exit();
}
$data = mysqli_fetch_array($result, MYSQL_ASSOC);
$hash = hash('sha256', $data['SALT'] . hash('sha256', $password));
if ($hash != $data['PASSWORD']) {
header('Location: index.php?msg=error5');
exit();
}
else {
echo "Logged in.";
}
?>
If I try and log in, the error message ?msg=error5 gets thrown, which means that the passwords do not match, but I cannot see a problem in that part of my code. I believe it's telling me they do not match because it's not looking in the right table/for the right data, which must be something wrong with my INNER JOIN.
Any help would be much appreciated.
EDIT: I realise now that it would have made a lot more sense to have just put the password and salt in to MEMBERS for both records so I'm only selecting from one table, but the deed is done and I'm curious to see if this is possible.

Answer 1

Actually, I tried your structure and code on http://www.w3schools.com/sql/trysql.asp?filename=trysql_select_all and this works for me:
SELECT * from members M JOIN couples C ON uid_1=Uid or uid_2=uid
WHERE email='youremail';
Perhaps the problem is that in the table there is more than 1 couple a person is actually in?
Like:
uid_1, uid_2, pass
1 2 xxx
1 3 yyy
You should either limit it on database side to unique columns or change code, because if you don't, you might be getting more than 1 row as result and you would need to check every couple's password against particular UID.