Update upper row data with next row data that was detached partially from import.

select * from fredong

ID   DOCNAME  COL1  COL2  COL3
 1    File1   ABC   DEF   GH  
 2    File1   I     JKH   LMN 
 3    File2   DDD   RRR   GG  
 4    File2   G     YYY   PPP 
 5    File3   TTT   CCC   MMM 
 6    File4   WWW   QQQ   ZZZ

select                                     
  t1.id as id,
  t1.DocName as DocName,
  case
    when length(strip(t1.col1)) = 1 then ''
    else t1.col1
  end as col1,
  t1.col2 as col2,
  case
    when length(strip(t2.col1)) = 1 then
      concat(strip(t1.col3), t2.col1)
    else t1.col3
  end as col3
from
  fredong t1 left join
  fredong t2 on t1.DocName = t2.DocName and
                t1.id != t2.id

ID   DOCNAME  COL1  COL2  COL3
 1    File1   ABC   DEF   GHI 
 2    File1         JKH   LMN 
 3    File2   DDD   RRR   GGG 
 4    File2         YYY   PPP 
 5    File3   TTT   CCC   MMM 
 6    File4   WWW   QQQ   ZZZ

...
when length(trim(t1.col1)) < N then ...
...
when length(trim(t2.col1)) < N then ...
...

SELECT t1.id, t1.docname,t1.col1,t1.col2,t1.col3||t2.col1 AS col3, t2.col2 as col4, t2.col3 as col5
FROM (select * from fredong  WHERE docname IN (SELECT docname FROM fredong GROUP BY docname HAVING count(*) > 1)) t1
     INNER JOIN (SELECT * FROM fredong ) t2
       ON t1.docname = t2.docname
          AND t1.id < t2.id
UNION ALL
SELECT id, docname,col1,col2,col3,null,null FROM fredong WHERE docname IN (SELECT docname FROM fredong GROUP BY docname HAVING count(*) = 1)
 order by docname;