求解sql关联查询

Question

SELECT *
FROM `table_1`
WHERE `reply_parent` IN
    (SELECT `reply_id`
     FROM
       (SELECT `reply_id`
        FROM `table_1`
        WHERE `reply_article`=69
          AND `reply_type`=0
        LIMIT 0,
              5)AS t)
UNION
  (SELECT *
   FROM `table_1`
   WHERE `reply_article`=69
     AND `reply_type`=0
   LIMIT 0,
         5)
ORDER BY `create_time` DESC

上面这段查询会将“table_1”中信息查询出来，现在需要 用查询出来的结果集里username字段去关联table_2中的username字段。然后查询出来的结果 和第一次“table_1”查询出的结果集进行UNION

本人sql属入门级别，到了一大堆sql查询的时候就手足无措了,求解答。☺