mysql array selecting and sum values - mysql

Question

I own an array that three records containing:
value datetime
2 03/03/2015 14:34:00
4 03/03/2015 14:36:00
5 03/03/2015 13:34:00
I want to select the records that are on time 14 and sum them. In the above example would be 4 + 2 = 6
How can I do this?
$sql ="SELECT amperagem, data FROM tomada WHERE date(data) = DATE_SUB(CURDATE(), INTERVAL 1 DAY)";//
mysql_select_db('localiza');
$retval = mysql_query( $sql, $conn );
$num_rows = mysql_num_rows($retval);
while($row = mysql_fetch_array($retval, MYSQL_BOTH)){
$hour= substr(($row['data']),11, 2);

Answer 1

The sql is as simple as:
select sum(value) from tomada where hour(datetime) = 14;

Answer 2

You can actually get yourself the value through a single SQL query:
Be sure to read up on http://www.w3schools.com/sql/func_datepart.asp
So to get all records with a datetime containing 14:XX:XX
SELECT value FROM tomada WHERE HOUR(data) = 14;
Now simply get the rows like you did and retrieve the 'value' per row and add them up
$res = mysql_query( $sql, $conn );
$returnObjects = array();
if ($res == null) {
return $returnObjects;
}
while(!is_null($row = mysql_fetch_row($res))){
$returnObjects[] = $row;
}
$returnArray = mysql_fetch_array($returnObjects);
$sum = 0;
for($row = 0, $size = count($returnArray); $row < $size; $row++){
$sum += $returnArray[$row][0]; //Note 0 is the value you need
}
return $sum;
Note it can be done in less lines of codes with less steps but I find this helps reading what i'm doing. Also some additional checks if certain objects are NULL or values are invalid is recommended.