How can I add to an existing value in sql table - sql

Question

How can i add a value to an existing value in SQL example a value 4 in a column how can i add 2 to it to make it 6 not to update to 2 but to add the previous value and the new value to together here is my class php file
<?php
class data
{
public $d_count;
public $id;
private $conn;
private $table_name;
public function __construct($db)
{
$this->conn = $db;
$this->table_name= "tbl_data";
}
public function updateCount()
{
// $query = "UPDATE tbl_data SET d_count = ?, date_updated = ? WHERE id = ?";
$query = "UPDATE tbl_data SET d_count '+1' = ?, date_updated = ? WHERE id = ?";
$obj = $this->conn->prepare($query);
$this->d_count = htmlspecialchars(strip_tags($this->d_count));
$this->date_updated = htmlspecialchars(strip_tags($this->date_updated));
$this->id = htmlspecialchars(strip_tags($this->id));
$obj->bind_param("sss", $this->d_count, $this->date_updated, $this->id);
if ($obj->execute()) {
return true;
}
return false;
}
}
The line I commented out is what i used to test the code and it works fine but now i need to add the old value + new value together here is the code am using to update the table
<?php
$data = new data($connection);
if($_SERVER['REQUEST_METHOD'] === "POST"){
$newDCount = json_decode(file_get_contents("php://input"));
if(!empty($newDCount->d_count))
$data->id= $newDCount->id;
$data->d_count = $newDCount->d_count;
$data->date_updated = date('Y-m-d H:i:s', time());
if($apiata->updateCount()){
http_response_code(200);
echo json_encode(array(
"status" => 200,
"message" => "Success"
));
}
else
{
http_response_code(200);
echo json_encode(array(
"status" => 500,
"message" => "Failed"
));
}
}

Answer 1

To update the d_count to add to the existing d_count value, the query should look like:
$sql = 'UPDATE catalog SET d_count = d_count + ?, date_updated = ? WHERE id = ?';
The other handling should be ok as-is.