Shopping cart is not behaving as expected

i used following query
UPDATE `ntidwebmysql`.`customer` SET `cart` = '' WHERE `customer`.`customer_id` =7 LIMIT 1 ;

After the execution of this query, contents of the column cart from the customer table for customer id =7 were set to ''.

When I logged in as that user, again the contents of the shopping cart were populated from the UI and even database row was updated accordingly.

Can you please tell me why is this so?