Shell Programming and Scripting

 
file1.txt
 
1|ABC|3|jul|dhj
2|NHU|4|kil|eu
3|hjd|34|hfd|43

 
 
file2.txt
 
1||3|KING|dhj
2|NHU||k|
3|hjd|34|hd|43

 
file3.txt

1|ABC|3|KING|dhj
2|NHU||k|eu
3|hjd|34|hd|43

 
join -t"|" -11 -21 file1.txt file2.txt
1|ABC|3|jul|dhj||3|KING|dhj
2|NHU|4|kil|eu|NHU||k|
3|hjd|34|hfd|43|hjd|34|hd|43

OK, have a try with pseudo-multidimensional arrays in awk:
awk 'NR==FNR{for(i=2; i<=NF; i++) A[$1,i]=$i; next} {for(i=2; i<=NF; i++) if($i=="") $i=A[$1,i]}1' FS=\| OFS=\| file1 file2

awk '
  NR==FNR{                                        # while reading the first file
    for(i=2; i<=NF; i++) A[$1,i]=$i               # record every field and tie it to the value of the first field..
    next                                          # when processing the first file then go on to read the next record
  }
  {                                               # when processing the second file
    for(i=2; i<=NF; i++) if($i=="") $i=A[$1,i]    # if a field number is empty, then look up the value of the first field and the relevant field number use this
  }
  1                                               # print the record...
' FS=\| OFS=\| file1 file2