Mysql - 简单添加到购物车数据库正在变得复杂。

I am stuck in a situation, I have three tables customer , stock , trolley. In customer I have fields id (pk) , username, password, date of birth.

In stock I have fields stock number , quantity , id (pk). While in the trolley I have fields cart_id (pk) ,customer_id (fk) ,paid.

Now My scenario is I want to store the stock selected by customer , in the trolley . But I dont know how it will happen. Stock table's id (pk) is auto increment and so does trolley's cart_id (pk) is auto increment. How can I store the stock selected by user and the cart id in separate table. In the end its my requirement that after saving the data , I need to store customer_id, cart_id and paid in trolley table too.