MySql - Product Variants Table (Wide Table) - Unique with NULLs - mysql

Question

I have a products table, and a product_variants table (one-to-many).
The product_variants table has the following structure:
CREATE TABLE product_variants (
id int(11) NOT NULL AUTO_INCREMENT,
id_product int(11) NOT NULL,
id_colourSet int(11) DEFAULT NULL,
id_size int(11) DEFAULT NULL,
PRIMARY KEY (id),
UNIQUE KEY UNIQUE (id_product,id_colourSet,id_size),
KEY idx_prod (id_product),
KEY idx_colourSet (id_colourSet),
KEY idx_size (id_size),
CONSTRAINT fk_df_product_variants_id_colurSet FOREIGN KEY (id_colourSet) REFERENCES df_colour_sets (id_colourSet) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT fk_df_product_variants_id_product FOREIGN KEY (id_product) REFERENCES df_products (id) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT fk_df_product_variants_id_size FOREIGN KEY (id_size) REFERENCES df_sizes (id) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB
The options are known at compile-time. Each option is foreign-keyed to a dedicated table, and the unique key is the combination of all options.
I then insert products with an "ON DUPLICATE KEY UPDATE ..." statement, and if a variant already exists the query will use an existing variant.
The problem is that certain products do not have a color, nor a size. In this case the unique constraint fails and I insert lots of almost-empty rows in the product_variants table.
In order to solve this problem I am creating a "NULL" value for each option (e.g. "NO_COLOR", "NO_SIZE") in the respective option tables, and using that as the default value for the option columns in the product_variants table.
Would this be the recommended solution? Is there a better way of structuring this data? I would really like to avoid an EAV design.
Thank you

Answer 1

Designating a magic value that means "missing value" is not the right solution in almost every case. That's what NULL is for.
It's also not clear how "NO_COLOR" is used for an integer. I guess it would map to the value 0, which is typically not used in an auto-increment column.
You can create another column to be a hash of the three unique key columns, defaulted to '' to avoid null problems. Then put a unique constraint on that hash.
CREATE TABLE product_variants (
id int(11) NOT NULL AUTO_INCREMENT,
id_product int(11) NOT NULL,
id_colourSet int(11) DEFAULT NULL,
id_size int(11) DEFAULT NULL,
option_hash binary(16) NOT NULL,
PRIMARY KEY (id),
UNIQUE KEY (option_hash),
KEY idx_prod (id_product),
KEY idx_colourSet (id_colourSet),
KEY idx_size (id_size),
CONSTRAINT fk_df_product_variants_id_colurSet FOREIGN KEY (id_colourSet) REFERENCES df_colour_sets (id_colourSet) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT fk_df_product_variants_id_product FOREIGN KEY (id_product) REFERENCES df_products (id) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT fk_df_product_variants_id_size FOREIGN KEY (id_size) REFERENCES df_sizes (id) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB;
CREATE TRIGGER product_variants_ins BEFORE INSERT ON product_variants
FOR EACH ROW SET option_hash = UNHEX(MD5(CONCAT_WS('|',
COALESCE(id_product, ''),
COALESCE(id_colourSet, ''),
COALESCE(id_size, ''))));
CREATE TRIGGER product_variants_upd BEFORE UPDATE ON product_variants
FOR EACH ROW SET option_hash = UNHEX(MD5(CONCAT_WS('|',
COALESCE(id_product, ''),
COALESCE(id_colourSet, ''),
COALESCE(id_size, ''))));