Designing a SQL schema for a combination of many-to-many relationship (variations of products)

Applying normalization to your problem, the solution is as given. Run and see it on SQL Fiddle.

CREATE TABLE products (
    product_id  int AUTO_INCREMENT PRIMARY KEY,
    name        varchar(20),
    description varchar(30)
);

INSERT INTO products
    (name, description)
VALUES
    ('Rug', 'A cool rug' ),
    ('Cup', 'A coffee cup');

-- ========================================

CREATE TABLE variants (
    variant_id int AUTO_INCREMENT PRIMARY KEY,
    variant    varchar(50)
);

INSERT INTO variants
    (variant)
VALUES
    ('color'),
    ('material'),
    ('size');

-- ========================================

CREATE TABLE variant_value (
    value_id   int AUTO_INCREMENT PRIMARY KEY,
    variant_id int,
    value      varchar(50)
);

INSERT INTO variant_value
    (variant_id, value)
VALUES
    (1, 'red'),
    (1, 'blue'),
    (1, 'green'),
    (2, 'wool'),
    (2, 'polyester'),
    (3, 'small'),
    (3, 'medium'),
    (3, 'large');

-- ========================================

CREATE TABLE product_variants (
    product_variants_id int AUTO_INCREMENT PRIMARY KEY,
    product_id          int,
    productvariantname  varchar(50),
    sku                 varchar(50),
    price               float
);

INSERT INTO product_variants
    (product_id, productvariantname, sku, price)
VALUES
    (1, 'red-wool', 'a121', 50),
    (1, 'red-polyester', 'a122', 50);

-- ========================================

CREATE TABLE product_details (
    product_detail_id   int AUTO_INCREMENT PRIMARY KEY,
    product_variants_id int,
    value_id            int
);

INSERT INTO product_details
    (product_variants_id, value_id)
VALUES
    (1, 1),
    (1, 4),
    (2, 1),
    (2, 5);