php mysql高手请进，帮助解释下面语句的含义，月详细越好
SELECT m.username as gy_user, m.password as gy_pw, m.styleid, m.newpm, u.*, u.specifiedusers LIKE '%\t$username\t%' AS specifieduser FROM $table_members m LEFT JOIN $table_usergroups u ON u.specifiedusers LIKE '%\t$username\t%' OR (u.status=m.status AND ((u.creditshigher='0' AND u.creditslower='0' AND u.specifiedusers='') OR (m.credit>=u.creditshigher AND m.credit<u.creditslower)))
WHERE username='$username' AND password='$password' ORDER BY specifieduser DESC
