How to create database within a database(postgres)? - database

Question

Actually I'm noob and stuck on this problem for a week. I will try explaining it.
I have table for USER,
and a table for product
I want to store data of every user for every product. Like if_product_bought, num_of_items, and all.
So only solution I can think of database within database , that is create a copy of products inside user named database and start storing.
If this is possible how or is there any other better solution
Thanks in advance

Answer 1

You actually don't create a database within a database (or a table within a table) when you use PostgreSQL or any other SQL RDBMS.
You use tables, and JOIN them. You normally would have an orders table, together with an items_x_orders table, on top of your users and items.
This is a very simplified scenario:
CREATE TABLE users
(
user_id INTEGER /* SERIAL */ NOT NULL PRIMARY KEY,
user_name text
) ;
CREATE TABLE items
(
item_id INTEGER /* SERIAL */ NOT NULL PRIMARY KEY,
item_description text NOT NULL,
item_unit text NOT NULL,
item_standard_price decimal(10,2) NOT NULL
) ;
CREATE TABLE orders
(
order_id INTEGER /* SERIAL */ NOT NULL PRIMARY KEY,
user_id INTEGER NOT NULL REFERENCES users(user_id),
order_date DATE NOT NULL DEFAULT now(),
other_data TEXT
) ;
CREATE TABLE items_x_orders
(
order_id INTEGER NOT NULL REFERENCES orders(order_id),
item_id INTEGER NOT NULL REFERENCES items(item_id),
-- You're supposed not to have the item more than once in an order
-- This makes the following the "natural key" for this table
PRIMARY KEY (order_id, item_id),
item_quantity DECIMAL(10,2) NOT NULL CHECK(item_quantity <> /* > */ 0),
item_percent_discount DECIMAL(5,2) NOT NULL DEFAULT 0.0,
other_data TEXT
) ;
This is all based in the so-called Relational Model. What you were thinking about is something else called a Hierarchical model, or a document model used in some NoSQL databases (where you store your data as a JSON or XML hierarchical structure).
You would fill those tables with data like:
INSERT INTO users
(user_id, user_name)
VALUES
(1, 'Alice Cooper') ;
INSERT INTO items
(item_id, item_description, item_unit, item_standard_price)
VALUES
(1, 'Oranges', 'kg', 0.75),
(2, 'Cookies', 'box', 1.25),
(3, 'Milk', '1l carton', 0.90) ;
INSERT INTO orders
(order_id, user_id)
VALUES
(100, 1) ;
INSERT INTO items_x_orders
(order_id, item_id, item_quantity, item_percent_discount, other_data)
VALUES
(100, 1, 2.5, 0.00, NULL),
(100, 2, 3.0, 0.00, 'I don''t want Oreo'),
(100, 3, 1.0, 0.05, 'Make it promo milk') ;
And then you would produce queries like the following one, where you JOIN all relevant tables:
SELECT
user_name, item_description, item_quantity, item_unit,
item_standard_price, item_percent_discount,
CAST(item_quantity * (item_standard_price * (1-item_percent_discount/100.0)) AS DECIMAL(10,2)) AS items_price
FROM
items_x_orders
JOIN orders USING (order_id)
JOIN items USING (item_id)
JOIN users USING (user_id) ;
...and get these results:
user_name | item_description | item_quantity | item_unit | item_standard_price | item_percent_discount | items_price
:----------- | :--------------- | ------------: | :-------- | ------------------: | --------------------: | ----------:
Alice Cooper | Oranges | 2.50 | kg | 0.75 | 0.00 | 1.88
Alice Cooper | Cookies | 3.00 | box | 1.25 | 0.00 | 3.75
Alice Cooper | Milk | 1.00 | 1l carton | 0.90 | 5.00 | 0.86
You can get all the code and test at dbfiddle here