Which date/paper pairs are NOT represented?

Okay, y'all have been very helpful so far so here's another difficult
query.

This is the same database I've asked about before:

papers: information about newspapers, including a unique paperid
letters: information and text of letters to the editor, including the
paperid

There are, as you might expect, numerous letters on a given date in a
given paper. What I need to know, though, is for what paper/date pairs in
my sample I do NOT have any letters (since this probably represents a
data-collection glitch). I know I can do:

SELECT paper, date, count(date) FROM papers, letters WHERE
papers.papercode = letters.papercode GROUP BY paper, date;

but adding a HAVING count(date) < 1 seems to return 0 rows.

Thanks-
ap

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Andrew J Perrin - andrew_perrin(at)unc(dot)edu - http://www.unc.edu/~aperrin
Assistant Professor of Sociology, U of North Carolina, Chapel Hill
269 Hamilton Hall, CB#3210, Chapel Hill, NC 27599-3210 USA