Grouping a set of measures in PHP

Use group_concat that concatenates a sequence of fields according to your defined query and group. Group_concat is an aggregate function such as count (), sum (), etc.

MySQLi

$servidor = "localhost";
$usuario = "USUARIO";
$senha = "SENHA";
$dbname = "NOME_DB";

$conn = mysqli_connect($servidor, $usuario, $senha, $dbname);
    $query = ('SELECT comprimento, group_concat(largura SEPARATOR " * ") AS lista, SUM(largura) as largura_sum FROM arquivos GROUP BY comprimento');
$result = mysqli_query($conn, $query);
while ($row = mysqli_fetch_assoc($result)){
    echo $row['comprimento']." - ";
    echo $row['lista']." Total: ";
    echo $row['largura_sum']."<br>";
}

PDO

$hostname="localhost";  
$username="USUARIO";  
$password="SENHA";  
$db = "NOME_DB";  
$dbh = new PDO("mysql:host=$hostname;dbname=$db", $username, $password);  
foreach($dbh->query('SELECT comprimento, group_concat(largura SEPARATOR " * ") AS lista, SUM(largura) as largura_sum FROM arquivos GROUP BY comprimento') as $row) {  

echo $row['comprimento'] . " - ";  
echo $row['lista']." Total: ";  
echo $row['largura_sum']."<br>"; 

} 

  

Generally, GROUP_CONCAT separates items with a comma:

item1,item2,item3,item4

  

You can set the tab to be used, like this:

GROUP_CONCAT(suacoluna SEPARATOR ' ')

Edited response to answer author's request for sum of widths

You can concatenate the columns representing the dimensions and group the data:

Table:

create table test.pedidos
(
    id_pedido int auto_increment primary key,
    comprimento float null,
    largura float null,
    data date null
);

Some data:

INSERT INTO pedidos
  (comprimento, largura, data)
VALUES
  (320, 20, '2017-05-05'),
  (320, 25, '2017-05-05'),
  (310, 10, '2017-05-06'),
  (320, 25, '2017-05-06'),
  (320, 20, '2017-05-07'),
  (320, 20, '2017-05-08'),
  (300, 100, '2017-05-10')
;

And the query with the information you need:

SELECT
  concat(comprimento, '-', largura),
  count(1)
FROM pedidos
GROUP BY concat(comprimento, '-', largura);