如何一起获取mysql查询结果

Question

我在mysql表size1, size2, size3, size4, size5中有5列。现在问题是我想将这些列混合在一起并将其显示为一个。

//输出

{"size":[{"size1":"0","size2":"0","size3":"0","size4":"0","size5":"0"},{"size1":"38","size2":"40","size3":"42","size4":"0","size5":"0"},{"size1":"38","size2":"40","size3":"44","size4":"0","size5":"0"}]}

//必需的输出

{"size":[{"size":"38"}, {"size":"40"}, {"size":"42"}, {"size":"44"}]}

我的代码是，

$query  =   "SELECT size1, size2, size3, size4, size5 FROM tbl_first_page_products UNION SELECT size1, size2, size3, size4, size5 FROM tbl_third_page_products UNION SELECT size1, size2, size3, size4, size5 FROM tbl_fourth_page_products";
$result =   mysqli_query($c, $query) or die(mysqli_error($c));
$length =   mysqli_num_rows($result);

if($length > 0)
{
    while($obj = mysqli_fetch_assoc($result)) 
    {
        $var[] = $obj;
    }
    $var = json_encode($var);
    echo '{"size":'.$var.'}';
}
else
{
    $arr    =   array('status'=>"notfound");
    echo '{"size":['.json_encode($arr).']}';
}