SQL可以对更新执行计算吗？

Question

Can SQL perform calculation on update? For instance, the stored product quantity was 5 , and after being sold, say 3 items, I want to update the quantity left in the product tables ,

    $sql_update = "
    UPDATE store_products
    SET 
        product_quantity = ?
    WHERE store_products.product_id = ?
    ";

    $result = $connection->run_query($sql_update,array(
        3,
        $product->product_id
));

Is it possible?

Answer 1

Yes - to achieve what you describe you need to implement an AFTER UPDATE trigger.

For details and samples see:

• http://dev.mysql.com/doc/refman/5.0/en/create-trigger.html
• http://www.roseindia.net/sql/trigger/mysql-trigger-after-update.shtml

EDIT - as per comment a sample:

CREATE TRIGGER produpd AFTER UPDATE ON store_products
  FOR EACH ROW BEGIN
    UPDATE product SET quantity = quantity - NEW.product_quantity WHERE product_id = NEW.product_id;
  END;

This trigger needs to be created in your MySQL DB... when you execute the SQL code from your question MySQL invokes this trigger and update the products table.