EAV model - How to restrict product properties? - mysql

Question

I have the following structure of my database, implementing a simple EAV model (see pic):
My product has a type, which through the junction table restricts prop_names, available for this product. And here everything is clear.
BUT:
Then I've added a prop_values table to keep the properties values for each product. It has reference to products through prod_sku and to prop_names through prop_id. And here the problem comes: One can add to any product any properties - even those, which are not allowed for this product type. Also, there can be duplications - two or more same properties for a single product.
Is there any way to restrict this on the database level?
After the #BillKarvin's answer, I've tried the below CREATE code, but failed with the 'Foreign key constraint is incorrectly formed' error when creating the last table (property_values).
I have found my error - I forgot to add a KEY to the products table. Below is the corrected (working) version of my code:
CREATE TABLE product_types (
id INT PRIMARY KEY,
product_type varchar(50) NOT NULL,
block_css_id varchar(50) NOT NULL,
block_description varchar(50) NOT NULL
);
CREATE TABLE products (
sku varchar(50) PRIMARY KEY,
name varchar(50) NOT NULL,
price decimal(20,2) unsigned NOT NULL,
id_product_type INT NOT NULL,
FOREIGN KEY (id_product_type) REFERENCES product_types (id),
KEY (sku, id_product_type)
);
CREATE TABLE property_names (
id INT PRIMARY KEY,
property_name varchar(50) NOT NULL,
property_css_id varchar(50) NOT NULL,
property_input_name varchar(50) NOT NULL
);
CREATE TABLE junction_ptype_propname (
id_productt_type INT NOT NULL,
id_property_name INT NOT NULL,
PRIMARY KEY (id_productt_type, id_property_name),
FOREIGN KEY (id_productt_type) REFERENCES product_types (id),
FOREIGN KEY (id_property_name) REFERENCES property_names (id)
);
CREATE TABLE property_values (
id INT NOT NULL PRIMARY KEY,
product_sku varchar(50) NOT NULL,
property_id INT NOT NULL,
property_value decimal(20,2) NOT NULL DEFAULT 0.00,
id_prod_type INT NOT NULL,
UNIQUE KEY (product_sku, property_id),
FOREIGN KEY (product_sku, id_prod_type) REFERENCES products (sku, id_product_type),
FOREIGN KEY (property_id, id_prod_type) REFERENCES junction_ptype_propname (id_property_name, id_productt_type)
);

Answer 1

I would design this in the following way:
There are few important differences from your model:
prop_values has a unique key on (prod_sku, prop_id) so you can only have one instance of a given property per product sku.
prop_values has a prod_type column, and this references products, using both columns (sku, prod_type).
prop_values has a compound foreign key to junction_ptype_propname instead of prop_name.
Now the prod_type in prop_values can have a single value per row, and it must reference the correct product type in both the products table and the junction_ptype_propname table. So it is constrained to be a valid property for the given product, and a valid property for the product type. You therefore cannot add a property to a product that isn't legitimate for that product's type.
Here's the DDL:
create table prod_types (
id int primary key,
type_name varchar(30) not null
);
create table products (
sku varchar(30) primary key,
name varchar(30) not null,
type int not null,
foreign key (type) references prod_types(id),
key(sku, type)
);
create table prop_names (
id int primary key,
prop_name varchar(30) not null
);
create table junction_ptype_propname (
id_prop_name int not null,
id_prod_type int not null,
primary key (id_prop_name, id_prod_type),
foreign key (id_prod_type) references prod_types(id),
foreign key (id_prop_name) references prop_names(id)
);
create table prop_values (
id int primary key,
prod_sku varchar(30) not null,
prod_type int not null,
prop_id int not null,
prop_value decimal not null,
unique key (prod_sku, prop_id),
foreign key (prod_sku, prod_type) references products(sku, type),
foreign key (prop_id, prod_type) references junction_ptype_propname(id_prop_name, id_prod_type)
);
This question is fun because it's a case of using Fifth Normal Form. Many articles on database design claim that normal forms past the Third Normal Form aren't used. But your model disproves that.

Answer 2

Also, there can be duplications - two or more same properties for a
single product.
Use UNIQUE to prevent from duplications
w3schools.com - UNIQUE