MYSQL reference to another table fields - mysql

Question

I am trying to get for a specific period of time the quantity of products sold. For this example : the products table has a quantity field which increments whenever a certain product is sold. The orders table has a start date and end date. Is there a way i can hold a reference of the products table (like an object) in my orders table so i can see the quantity sold for each product every start - end date ? A quick example : I am having 2 products:
1 bike 25000 3 and 2 sportbike 30000 5 sold for my date. So an order would be something like : 1 05.07.2015 05.07.2015 and those products.
CREATE TABLE products (
product_no integer PRIMARY KEY,
name varchar(20),
price float,
quantity integer,
);
CREATE TABLE sells (
sell_id integer PRIMARY KEY,
start_date date,
end_date date
);
//New idea :
CREATE TABLE products (
product_no integer PRIMARY KEY,
name varchar(20),
price float
);
CREATE TABLE sells (
sell_id integer PRIMARY KEY,
date date,
quantity integer,
product_id integer FOREIGN KEY
);

Answer 1

Unless an order is always for a single product, I think you will need a third table. To me a minimal *) data model for this situation would look like this:
CREATE TABLE products (
product_no integer PRIMARY KEY,
name varchar(20) not null,
price float,
quantity_on_stock integer not null
);
CREATE TABLE orders (
order_id integer PRIMARY KEY,
order_date date not null
);
CREATE TABLE orderlines (
order_id integer not null REFERENCES orders.order_id,
product_no integer REFERENCES products.product_no,
price integer,
quantity integer not null,
PRIMARY KEY(order_id, product_no)
);
Then, a query to get sales in a certain period could look like this:
select
p.product_no,
p.name,
sum(ol.quantity)
from
products p
inner join orderlines ol on ol.product_no = p.product_no
inner join orderlines o on o.order_id = ol.order_id
where
ol.order_date between :start_date and :end_date
*) I say minimal, but it's actually less than minimal. You'd probably want to store invoices too, or at least some kind of indication that says whether an order is actually payed for and delivered, since sometimes orders are cancelled, or are just waiting open.

Answer 2

You definitely want a sales table showing what was sold and when, rather than a table showing the running inventory.
It is much easier in SQL to aggregate detail to create running totals than it is to reconstruct detail from running totals.
So, if you happen to have a sales table containing, among other things, columns for product_id, quantity, and ts (the timestamp of the sale), then you can get a summary of sales by product_id and sales date like this.
SELECT product_id,
DATE(ts) AS salesdate,
SUM(quantity) AS quantity
FROM sales
WHERE ts >= :start_date
AND ts < :end_date + INTERVAL 1 DAY
GROUP BY DATE(ts), product_id
One of the things that's cool about this is you can add a column for transaction_type to this table. If a customer returns a product you tag it return and use a negative quantity. If your shop receives a shipment of products you tag it restock and use a negative quantity. Then, if you want to show net sales -- sales less returned products -- it's a small change to the query.
SELECT product_id,
DATE(ts) AS salesdate,
SUM(quantity) AS quantity
FROM sales
WHERE ts >= :start_date
AND ts < :end_date + INTERVAL 1 DAY
AND transaction_type IN ('sale', 'return')
GROUP BY DATE(ts), product_id
If you want to know how much stock you had on on hand for each product for a particular end_date, you do this (starting from the beginning of time).
SELECT product_id,
SUM(quantity) AS quantity
FROM sales
GROUP BY product_id
In actual inventory / sale systems, it's common to take inventory once a year, close out the previous year's sales table, and start a new one with a starting value for each product. it's also common to organize sales by sales_order and sales_detail, so you can track the sale of multiple items in each transaction.