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$sql = qq{select course_id,course_name,course_sitting_id,course_sittin
+g_date from courses};
$sth = $dbh->prepare($sql) or die . . .;
$sth->execute() or die  . . . .;
while (@fetch = $sth->fetchrow)
        { . . . .

10,Programming 101,1,01-SEP-2011
20,Databases 101,1,03-SEP-2011
10,Programming 101,2,20-SEP-2011
20,Databases 101,2,27-SEP-2011

my %courses= (
    '10' => {
...
        },
    },
);