#!/usr/bin/perl -w
use strict;
use DBI;

my @data = (
['2004-02-01 07:01',   82],
['2004-02-01 11:38',  172],
['2004-02-01 22:48',  154],
['2004-02-02 05:38',  107],
['2004-02-02 13:20',  117],
['2004-02-02 23:48',  188]);

my $dbh = DBI->connect("dbi:SQLite:averages","","")
    or die $DBI::errstr;

unless ( -s "averages") {
    $dbh->do(qq{ CREATE TABLE averages (
                d datetime not null,
                val INTEGER)})
                or die $DBI::errstr;
    for (@data) {
        $dbh->do( qq{INSERT INTO averages VALUES (?, ?)}, undef, @$_)
            or die $DBI::errstr
    }
}

my $query = qq{
 SELECT
    substr(d,1,10) as day,
    CASE
        WHEN substr(d,12,2) BETWEEN  4 AND 12 THEN "1 morning"
        WHEN substr(d,12,2) BETWEEN 13 AND 18 THEN "2 afternoon"
        WHEN substr(d,12,2) BETWEEN 19 AND 24 THEN "3 evening"
        ELSE "4 night"
    END AS tm,
    AVG(val)
FROM
    averages
GROUP BY
    day, tm
ORDER BY
    day, tm
};

my $averages = $dbh->selectall_arrayref($query)
    or die $DBI::errstr;
printf  "%-10s %-15s %5.2f\n", @$_ for @$averages;
__END__
output:
2004-02-01 1 morning       127.00
2004-02-01 3 evening       154.00
2004-02-02 1 morning       107.00
2004-02-02 2 afternoon     117.00
2004-02-02 3 evening       188.00