REPLACE and SET in mysql query - mysql

Question

According to w3resource you can replace text using REPLACE keyword.
Now look at this code extracted from OpenCart
<?php
class ModelToolOnline extends Model {
public function addOnline($ip, $customer_id, $url, $referer) {
$this->db->query("DELETE FROM `" . DB_PREFIX . "customer_online` WHERE date_added < '" . date('Y-m-d H:i:s', strtotime('-1 hour')) . "'");
$this->db->query("REPLACE INTO `" . DB_PREFIX . "customer_online` SET `ip` = '" . $this->db->escape($ip) . "', `customer_id` = '" . (int)$customer_id . "', `url` = '" . $this->db->escape($url) . "', `referer` = '" . $this->db->escape($referer) . "', `date_added` = '" . $this->db->escape(date('Y-m-d H:i:s')) . "'");
}
}
?>
They have Replace query with SET keyword and no WHERE condition. Now I couldn't find anything similar to this anywhere to understand whats happening in this query.
If anyone knows what is happening in this query please explain with authentic source link

Answer 1

That are two different things. There is a REPLACE() function and a REPLACE statement.
In the code the REPLACE statement is used. It is similar to an INSERT statement, and thus has no WHERE clause.
Here's what the documentatin says:
REPLACE works exactly like INSERT, except that if an old row in the
table has the same value as a new row for a PRIMARY KEY or a UNIQUE
index, the old row is deleted before the new row is inserted.