How to represent order content in SQL [closed] - sql

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I am creating a pizza shipping website, and I need to represent the orders and their content somewhere in the database.
The problem is, I have absolutely no idea how to store items and their quantities.
My first thought is to create an order_content table, which contains the order's id as a foreign key, and has two extra columns : item id and quantity.
Another problem : I have multiple types of items : pizzas, drinks, extras etc... So id's aren't unique across categories. I can't just say, for example in order_content item_id 1, quantity 1, because item_id 1 can mean drink with id 1, pizza with id 1, etc...
Another big problem : I have a custom pizza which can have 3 to 6 custom ingredients. I have ingredients in a table with their unique id's... How can I represent this custom pizza in orders ?
Thank you
PS : I am fairly a beginner in SQL - Relational databases

Answer 1

Here is a schema that you should experiment with and modify to suit your needs.
create table clients(
id serial,
name varchar(25) not null,
email varchar(25) not null,
telephone varchar(25),
constraint pk_clients_id primary key (id),
constraint uq_clients_email unique(email)
);
create table items(
id serial,
name varchar(25),
price decimal(5,2),
constraint pk_items_id primary key (id)
);
create table orders(
id serial,
client_id int,
constraint pk_orders_id primary key (id),
constraint fk_orders_client foreign key (client_id) references clients(id)
);
create table toppings(
id serial,
name varchar(25) not null,
constraint pk_topping primary key (id));
create table order_details(
order_id int,
item_id int,
quantity int,
topping_1 int,
topping_2 int,
topping_3 int,
topping_4 int,
topping_5 int,
topping_6 int,
constraint fk_order_details_order_id foreign key (order_id) references orders(id),
constraint fk_custom_pizza_id foreign key (item_id) references items(id),
constraint pk_order_details_order_item_ids primary key(order_id, item_id)
);
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insert into clients (name, email, telephone) values ('Andrew','andrew#gmail.com','0123456789');
insert into items (name, price) values('custom pizza','20.00'),('1.5 litre coca-cola',5);
insert into toppings (name) values('mozzarella'),('parma ham'),('mushrooms'),('olives'),('red peppers'),('salmon');
1 rows affected
2 rows affected
6 rows affected
with order_number as
(insert into orders (client_id) values (1)
returning id)
insert into order_details
select order_number.id,1,1,1,2,3,4,5,6 from order_number
union all
select order_number.id,2,1,null,null,null,null,null,null from order_number;
2 rows affected
select
c.id,
c.name,
o.id order_number,
od.item_id,
i.name,
i.price,
od.quantity * i.price as line_total,
t1.name topping_1,
t2.name topping_2,
t3.name topping_3,
t4.name topping_4,
t5.name topping_5,
t6.name topping_6
from clients c
left join orders o on c.id = o.client_id
left join order_details od on o.id = od.order_id
left join items i on od.item_id = i.id
left join toppings t1 on od.topping_1 = t1.id
left join toppings t2 on od.topping_2 = t2.id
left join toppings t3 on od.topping_3 = t3.id
left join toppings t4 on od.topping_4 = t4.id
left join toppings t5 on od.topping_5 = t5.id
left join toppings t6 on od.topping_6 = t6.id
id | name | order_number | item_id | name | price | line_total | topping_1 | topping_2 | topping_3 | topping_4 | topping_5 | topping_6
-: | :----- | -----------: | ------: | :------------------ | ----: | ---------: | :--------- | :-------- | :-------- | :-------- | :---------- | :--------
1 | Andrew | 1 | 1 | custom pizza | 20.00 | 20.00 | mozzarella | parma ham | mushrooms | olives | red peppers | salmon
1 | Andrew | 1 | 2 | 1.5 litre coca-cola | 5.00 | 5.00 | null | null | null | null | null | null
db<>fiddle here

Answer 2

In general, storage systems of shopping sites are using relations between an order and an order line.
You could organize your DB like this and answer your problems
order
order_line
product
ingredient
ingredient_for_product
id
order_id
product_id
ingredient_id
ingredient_id
...
quantity
current_unit_price
name
order_line_id
unit_price
product_type
additional_price
quantity
product_id
product is an abstract concept that holds all sold products by your company. If you need to be more precise, you can either add fields that will be completed depending on the value of product_type or create another table with a one-on-one relationship with the product table.
you have a unit_price in the order_line table and a current_unit_price in the product table.
This has two uses :
if you change the price of your product a posteriori, you will still keep the price your customer paid
it allows you to store a price which is different from your registered "current_unit_price". For example, adding the value of supplementary ingredients

Answer 3

You might want to take a look at the fact table and dimension table definitions. Once this concept is understood it will be clear for you the the order table will be a fact table, and tables such as ingredients will be a dimension table.