书
| B_ID | Name | Unit_Price|
|------+---------+-----------|
| B01 | Math | 25 |
| B02 | Science | 34 |
命令
| O_ID | B_ID | Quantity |Total_Price|
|------+------+-----------+-----------|
| O01 | B01 | 2 | ? |
| O02 | B02 | 5 | ? |
如何通过乘以FK 来获得Total_Price到. 谢谢!Order.Quantity
Book.Unit_Price
Order.B_ID
Book.B_ID