在更新MySql，PHP上将值从一个表插入到另一个表

Question

I was wondering if there was a way to take one value from a column in a Table and add it to a column in another table.

The Scenario: I have a table (Shopping_Cart). It stores all of my customers shopping cart items in the columns CustomerID , PID , Price , Status (Status can be "IN_CART" which means it was added and "OPEN" means that it has been paid for and ready to be processed and shipped). Then I have a table (Products). It stores product information in columns PID , Price , Weight`. When My customers place an item in the shopping cart I do not capture the price. Prices can change daily so if they add something today, when they come back tomorrow the price may increase or decrease in their shopping cart. When my customer checks out and payment is confirmed, I am trying to grab the Price in the (Products) table where Products.PID = Shopping_Cart.PID and then apply that price to the Shopping_Cart.Price to lock in that final price since the customer has paid for it. Any Ideas? I got it set up to change the items for that customers cart to "OPEN" but I am a little lost on getting my script to grab a value from one table and applying it to another table. Below is my PHP Script for changing the Lines from "IN_CART" to "OPEN".

if($The_Function=="CLOSE_LINES"){

    $cart_close_session_id = $_GET['close_session_id'];

    $response = array();


    require_once __DIR__ . '/db_connect.php';

    //connect to my db
    $db = new DB_CONNECT();

    $result = mysql_query("UPDATE shopping_cart SET Status = 'OPEN' WHERE SessionID LIKE '$cart_close_session_id' AND Status LIKE 'IN_CART'");

    if ($result) {
        $response["close_success"] = 1;

//GRAB PRICE FROM "PRODUCTS TABLE" AND UPDATE SHOPPING_CART 

    }else{
        $response["close_success"] = 0;
    }
    echo json_encode($response);
}

Answer 1

Assuming that you have a *shopping_cart* table with a price and a product_id column, you can update it with the price of the products table, like this:

UPDATE shopping_cart SET price = 
    (SELECT price FROM products WHERE id = shopping_cart.product_id)

Answer 2

You can join against the products table and update the shopping_cart upon purchase as seen below:

UPDATE shopping_cart c
  JOIN products p 
    ON p.pid = c.pid
   SET c.price = p.price
 WHERE c.customer_id = ?