plan9-join(1) - relational database operator
-a n   In addition to the normal output, produce a line for each unpairable line in file n, where n is  1
       or 2.
-v n   Like -a, omitting output for paired lines.
-e s   Replace empty output fields by string s.
-1 m
-2 m   Join on the mth field of file1 or file2.
-jn m  Archaic equivalent for -n m.
-ofields
       Each  output  line  comprises  the  designated  fields.  The comma-separated field designators are
       either 0, meaning the join field, or have the form n.m, where n is a file number and m is a  field
       number.  Archaic usage allows separate arguments for field designators.
-tc    Use  character c as the only separator (tab character) on input and output.  Every appearance of c
       in a line is significant.