mysql subquery returns multiple rows

Question

My problem is in the first subquery select. I am being told that I am returning multiple rows.

$sql = "SELECT
      messages.message_id
    , messages.sent_timestamp
    , messages.content
    , messages.subject
    , users.user_name
    , (SELECT thread_participants.user_id 
FROM thread_participants 
WHERE thread_participants.user_id !=".$user_id.") as thread_participants
    , (SELECT message_read_state.readDate
       FROM message_read_state
       WHERE message_read_state.message_id = messages.message_id
        and message_read_state.user_id =". $user_id.") as ReadState
    FROM (messages INNER JOIN users ON messages.sender_user_id = users.user_id
    INNER JOIN thread_participants tp ON tp.thread_id = messages.thread_id)
    WHERE (((messages.thread_id)=".$thread_id."))
    ORDER BY messages.sent_timestamp DESC";

Answer 1

One way I found of doing it:

$sql = "SELECT
      messages.message_id
    , messages.sent_timestamp
    , messages.content
    , messages.subject
    , users.user_name
    , tp.user_id as thread_participants
    , (SELECT users.user_name FROM users WHERE users.user_id = thread_participants && users.user_id != messages.sender_user_id) as member_names
    , (SELECT message_read_state.readDate
       FROM message_read_state
       WHERE message_read_state.message_id = messages.message_id
        and message_read_state.user_id =". $user_id.") as ReadState
    FROM (messages INNER JOIN users ON messages.sender_user_id = users.user_id
    INNER JOIN thread_participants tp ON tp.thread_id = messages.thread_id)
    WHERE (((messages.thread_id)=".$thread_id."))
    ORDER BY messages.sent_timestamp DESC";