Question

I have a table products and table sizes. Each product can have multiple sizes. How can I design my database in order to not have repeated entries of the same product?

Answer 1

The typical approach to a many-many relationship is to have a mapping table called Product_Size that contains the Primary Keys of each table.

create table Product (
    id uniqueidentifier not null,
    name varchar(255),
    primary key (id))

create table Size (
    id int,
    name varchar(255),
    primary key (id))

create table Product_Size (
    productId uniqueidentifier,
    sizeId int,
    primary key (productId, sizeId),
    foreign key (productId) references Product(id),
    foreign key (sizeId) references Size(id))