Struttura del carrello MySQL

Non sono troppo sicuro di quale sia il problema qui... Probabilmente creerei quattro tabelle:

-- a table of item types (t-shirt, birthday card, etc.)
CREATE TABLE ItemTypes (
  TypeID      INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  TypeName    VARCHAR(20) NOT NULL
);

-- a table of associated properties
CREATE TABLE TypeProperties (
  PropertyID  INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  TypeID      INT NOT NULL,
  PropName    VARCHAR(20) NOT NULL,
  INDEX(Property, TypeID),
  FOREIGN KEY(TypeID) REFERENCES ItemTypes(TypeID)
);

-- a table of specific items (XL Blue t-shirt, large bday card w/sports car, etc.)
CREATE TABLE Items (
  ItemID      INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  TypeID      INT NOT NULL,
  ItemName    VARCHAR(100) NOT NULL,
  ItemPrice   DECIMAL UNSIGNED NOT NULL,
  ItemStock   INT UNSIGNED NOT NULL,
  INDEX(ItemID, TypeID),
  FOREIGN KEY(TypeID) REFERENCES ItemTypes(TypeID)
);

-- the dictionary of property values
CREATE TABLE ItemProperties (
  ItemID      INT NOT NULL,
  TypeID      INT NOT NULL,
  PropertyID  INT NOT NULL,
  Value       VARCHAR(20) NOT NULL,
  PRIMARY KEY(ItemID, Property),
  INDEX(ItemID,     TypeID),
  INDEX(PropertyID, TypeID),
  FOREIGN KEY(            TypeID) REFERENCES ItemTypes     (            TypeID),
  FOREIGN KEY(ItemID,     TypeID) REFERENCES Items         (ItemID,     TypeID),
  FOREIGN KEY(PropertyID, TypeID) REFERENCES TypeProperties(PropertyID, TypeID)
);

Dovrebbe essere abbastanza ovvio, ma per ogni evenienza, i dati di esempio assomiglierebbero a:

INSERT INTO ItemTypes (TypeID, TypeName) VALUES
  (1, 'T-Shirt'      ),
  (2, 'Birthday Card'),
  (3, 'Balloon'      );

INSERT INTO TypeProperties(PropertyID, TypeID, PropName) VALUES
  (51, 1, 'Colour' ), (52, 1, 'Size'),
  (53, 2, 'Size/mm'), (54, 2, 'Size'), (55, 2, 'Design'),
  (56, 3, 'Colour' );

INSERT INTO Items (ItemID, TypeID, ItemName, ItemPrice, ItemStock) VALUES
  (101, 1, 'Extra Large Blue T-Shirt',                       10.99, 20),
  (102, 2, '6mm Large Birthday Card with Sports Car Design',  2.99, 17),
  (103, 1, 'Extra Large Black T-Shirt',                      10.99,  5),
  (104, 3, 'Pink balloon',                                    0.10, 60);

INSERT INTO ItemProperties (ItemID, TypeID, PropertyID, Value) VALUES
  (101, 1, 51, 'Blue'       ),
  (101, 1, 52, 'Extra Large'),
  (102, 2, 53, '6'          ),
  (102, 2, 54, 'Large'      ),
  (102, 2, 55, 'Sports Car' ),
  (103, 1, 51, 'Black'      ),
  (103, 1, 52, 'Extra Large'),
  (104, 3, 56, 'Pink'       );