I just uploaded a utility I wrote some time ago to handle this sort of thing on the command line. Look for "cmpcol" by graff in the code catacombs (new posting).

It probably doesn't do exactly what you want in your particular case, but it does tend to be very handy for a lot of things...

Here are some sample usages, given your left and right files as input:

# intersection (-i) of key fields:
# (default delimiter is \s+, so use "-d ," here)

$ cmpcol -i -d , left right
1
4
5

# print those records with the combined line
# content from both files:

$ cmpcol -i -d , -lb left right
1,a:<>:1,a
4,f:<>:4,g
5,h:<>:5,h

# keys only in "left" file, which is "#1" on the
# command line (-x1), showing full line (-l1):

$ cmpcol -x1 -l1 -d ,  left right
2,b

# lines with identical content (don't use
# the comma delimiter in this case):

$ cmpcol -i  left right
1,a
5,h

# all lines that are not identical (this
# always shows which file contained each line):

$ cmpcol -x left right
2,b <1
3,c <2
4,f <1
4,g <2

# and so on.  I think the "usage" message is
# the best part (at least I hope so):

$ cmpcol

Usage: cmpcol {-i|-u|-us|-x|-x1|-x2} [options] file1[:col#] file2[:col
+#]

 Comparison modes:
  -i        : produce intersection of file1[:col#] and file2[:col#]
  -u(s)     : produce union (and identify sources)
  -x        : produce exclusive-or, identifying sources
  -x1 (-x2) : produce items unique to file1 (or file2)

 Options:
  -l{1,2,b} : print whole lines from file1,file2 or both (default: pri
+nt column)
  -g(v) ptn : grep (-v) -- only compare lines that (don't) contain /pt
+n/
  -c cchar  : ignore material following cchar
  -d delim  : use /delim/ as column separator (default is white-space)
              use "-d tab" for tab-delimited, "-d dot" for period-deli
+mited

 file1 or file2 may be 'stdin'
 default ':col#' for comparison is first column from each file, which 
+is ':1'
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