Consider the following schema: suppliers (sid: integer, sname: string, street: string, city: string, zip: string) parts (pid: integer, pname: string, color: string) catalog (sid: integer, pid: integer, cost: real) the primary key attributes are underlined, and the domain of each attribute is listed after the attribute name. thus, sid is the primary key for suppliers, pid is the primary key for parts, and sid and pid together form the primary key for catalog. attribute sid in catalog is a foreign key that refers to the primary key sid of suppliers and pid in catalog is a foreign key that refers to the primary key pid of parts. the catalog relation lists the prices charged for parts by suppliers.