สอบถามปัญหาการ JOIN SQL ที่การดึงข้อมูลมันทับซ้อนกันครับ

select tp.*, m1.username as createname, m2.username as lastname 
from tbl_topic as tp 
LEFT JOIN tbl_member as m1 ON tp.create_by = m1.user_id
Left join tbl_member as m2 on tp.last_by=m2.user_id

SELECT GROUP_CONCAT(last_by SEPARATOR ',') FROM tbl_topic [WHERE .....] GROUP BY last_by 

SELECT user_id, fullname FROM tbl_member WHERE user_id IN (ไอดีที่ได้มา)

$sql = "select * from tbl_topic LEFT JOIN tbl_member ON (tbl_topic.create_by = tbl_member.user_id)";

$lastPostBy = isset($allUserFullname[$show_topic['last_by']]) ? $allUserFullname[$show_topic['last_by']] : $show_topic['last_by'];