Insert id retrieving data from different tables in SQL - sql

Question

I have a Table A with customers and a Table P with products.
I also have a table C which represents the purchased product by customers.
The table fields are as follow:
A (Customers)
user_id
Name
Surname
P (Products)
product_id
price
product name
C (Purchased products)
id
product_id
user_id
quantity
date
C is the many-to-many link between A and P.
Assume that today a customer named "Bob Wright" has bought a product called "Beautiful_magazine". Assume that both customer and product are already in the database.
How do I make the entry in the C table?
I know that I should use the insert into select statement but I am facing hard time because I should retrieve the user id form A and the product id form P and then placing them into a new line in C along with the function NOW() and a numerical values representing the price.
I cannot do that because I am a newbie.
Any hint?
EDIT
I would like to make it manually, using the INSERT statement without relying on the software.
Assume:
user_id: 890
Name: Bob
Surname: Wright
product_id: 4897
price: 5.90
product_name: Beautiful_magazine
I need to create a new line in C table as follow:
id (autoincremental)
product_id: 4897
user_id: 890
quantity: 1
date: '2017-2-20'
Obviously, the product_id and user_id from P and A should be retrieve through a select statement using the where statement.

Answer 1

If you are passing the data in from an application, use a procedure (assuming that the id auto increments)
CREATE PROCEDURE `AddSale` (IN ProdID INT, IN UserID INT, IN Qty INT)
BEGIN
insert into TableC (Product_ID, User_ID, Quantity, Date)
values (ProdID, UserID, Qty, Now());
END
In your application, call the procedure and passin the parameters