+-------------+ +------------------+
| Products    | | Supplier_Product |
+-------------+ +------------------+
| PNO | PNAME | | SNO     | PNO    |
+-----+-------+ +---------+--------+
| P1  | Screw | | S1      | P1     |
| P1  | Screw | | S1      | P1     |
| P1  | Screw | | S1      | P2     |
| P2  | Screw | +------------------+
+-------------+

##</code><code>##

SELECT parts.pno 
FROM   parts
WHERE  parts.pname = 'Screw'
   OR  parts.pno IN
      (SELECT supplier_parts.pno
       FROM   supplier_parts
       WHERE  supplier_parts.sno = 'S1')

##</code><code>##

SELECT supplier_parts.pno 
FROM   supplier_parts
WHERE  supplier_parts.sno = 'S1'
   OR  supplier_parts.pno IN
      (SELECT parts.pno
       FROM   parts
       WHERE  parts.pname = 'Screw')

##</code><code>##

SELECT parts.pno 
FROM   parts, supplier_parts
WHERE  ( supplier_parts.sno = 'S1' AND
         supplier_parts.pno = parts.pno )
   OR  parts.pname = 'Screw'

##</code><code>##

#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;
use DBI;
my $DATABASE = 'distinct.db';
my $P        = 'parts';
my $SP       = 'supplier_parts';

my $dbh = DBI->connect( "dbi:SQLite:dbname=$DATABASE", "", "" );

create_tables($dbh);
insert_data($dbh);

display_results(select_statements());
display_results(distinct_select_statements());

unlink $DATABASE or die "Could not unlink $DATABASE: $!";

sub display_results {
    my @sql_statements = @_;
    foreach my $select ( @sql_statements ) {
        my @results = @{ $dbh->selectall_arrayref($select) };
        my %results_for;
        foreach my $result (@results) {
            no warnings 'uninitialized';
            $results_for{$select}{ $result->[0] }++;
        }
        while ( my ( $sql, $results ) = each %results_for ) {
            print "$sql\n\tResults: P1: $results->{P1} P2: $results->{P2}\n\n    --\n";
        }
    }
}

sub create_tables {
    my $dbh = shift;
    $dbh->do("CREATE TABLE $P ( pno, pname )");
    $dbh->do("CREATE TABLE $SP ( sno, pno )");
}

sub insert_data {
    my $dbh = shift;
    $dbh->do("INSERT INTO $P VALUES ('P1', 'Screw')");
    $dbh->do("INSERT INTO $P VALUES ('P1', 'Screw')");
    $dbh->do("INSERT INTO $P VALUES ('P1', 'Screw')");
    $dbh->do("INSERT INTO $P VALUES ('P2', 'Screw')");

    $dbh->do("INSERT INTO $SP VALUES ('S1', 'P1')");
    $dbh->do("INSERT INTO $SP VALUES ('S1', 'P1')");
    $dbh->do("INSERT INTO $SP VALUES ('S1', 'P2')");
}

sub distinct_select_statements {
    my @statements = map { s/SELECT/SELECT DISTINCT/g; $_ } select_statements();
}

sub select_statements {
    my @statements;
    push @statements => <<"    END_SQL";
    SELECT $P.pno 
    FROM   $P
    WHERE  $P.pname = 'Screw'
    OR     $P.pno IN
        (SELECT $SP.pno
         FROM   $SP
         WHERE  $SP.sno = 'S1')
    END_SQL
    push @statements => <<"    END_SQL";
    SELECT $SP.pno 
    FROM   $SP
    WHERE  $SP.sno = 'S1'
    OR     $SP.pno IN
        (SELECT $P.pno
         FROM   $P
         WHERE  $P.pname = 'Screw')
    END_SQL
    push @statements => <<"    END_SQL";
    SELECT $P.pno 
    FROM   $P, $SP
    WHERE  ( $SP.sno = 'S1' AND
             $SP.pno = $P.pno )
       OR  $P.pname = 'Screw'
    END_SQL
    return @statements;
}

##</code><code>##

SELECT city 
FROM   cities, customers
WHERE  customers.city_id = cities.city_id
  AND  customers.credit_rating > 700