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Date: Thu, 3 Sep 1992 10:48:34 EDT
Reply-To: Andy Norton <76350.1604@COM.COMPUSERVE>
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From: Andy Norton <76350.1604@COM.COMPUSERVE>
Subject: Re: Finding non-dups via SQL (multiple keys)
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----------------------------------------------------------------------
CONTENT:    Information
SUMMARY:   Jim Clark's response to Ray Pass is correct for single
           keys.  Here are solutions for multiple keys.
REL/PLTF:  6.07.01/CMS, 6.04/PC-DOS
E-ADDR:    76350.1604@compuserve.com
NAME:      Andy Norton
ADDRESS:   Trilogy Consulting, 5228 Lovers Lane, Kalamazoo MI 49002
PHONE:     (616) 344-2191
----------------------------------------------------------------------

Ray Pass had asked for a method to identify CASENO's in FILE1 that do
not occur in FILE2.

Jim Clark responded with
> title 'Eval Courses NOT IN Class Courses';
> select distinct e.ident, e.ins
>    from work.check e, f.course c
>    where e.ident not in
>     (select c.ident
>      from f.course c)
>    order by e.ident;

The general approach is correct and sufficient for Ray's problem.  I
think you could use simply
  select    CHECK.*
  from      WORK.CHECK
  where     CHECK.IDENT not in
            ( select IDENT
              from   F.COURSE)
  order by  IDENT;
to get the same result.  In other words, it is not necessary to join E
with C in order to reference table C in a subquery.

The main purpose of my posting, however, is to point out that this
method falls apart when you need to match on two keys instead of one.
Suppose the IDENT's are repeated within different colleges, and you
need to do a combined analysis.  Now the observations are identified
with {COLLEGE, IDENT}.  The IN operator only works for a single
variable, so can't handle this situation.

This is a widespread weakness in the design of SQL:  programs that work
with single columns often cannot be extended to handle multiple
columns.  If you write the program to handle multiple keys, it will be
able to handle the single-key situation.

For a general solution which works with multiple keys, use
  select    *
  from      WORK.CHECK
  where     not exists
            ( select *
              from   F.COURSE
              where  COLLEGE = CHECK.COLLEGE and
                     IDENT   = CHECK.IDENT)
  order by  COLLEGE, IDENT;

  This is fairly intelligible but can be inefficient with the current
optimizer, because the subquery is independently evaluated for each set
of {COLLEGE, IDENT} values.  Defining indexes on COLLEGE and IDENT (in
the COURSE file) could help.

It may be possible for the optimizer to automatically convert
subqueries to joins in the future.

This alternative may be more efficient (depending on factors such as
how many distinct {COLLEGE, IDENT} sets there are).  I haven't tested
enough to determine exactly what the trade-offs are:

  select    CHECK.*
  from      WORK.CHECK,
            ( select COLLEGE, IDENT
              from   WORK.CHECK
              except
              select COLLEGE, IDENT
              from   F.COURSE) as NONDUP
  where     CHECK.COLLEGE = NONDUP.COLLEGE and
            CHECK.IDENT   = NONDUP.IDENT
  order by  COLLEGE, IDENT;

Unlike the earlier subquery solution, this program processes all of the
different {COLLEGE, IDENT} pairs together.

                    Andy

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