Overriding primary keys

Let's consider the following schema fragment (from PostgreSQL):

CREATE TABLE "customer" (
  "customer_number" BIGINT NOT NULL 
                         DEFAULT NEXTVAL ('"customer_seq"'),
  "customer_name" VARCHAR(50) NOT NULL, 
  ... 
  CONSTRAINT "customer_pk" PRIMARY KEY ("customer_number"), 
  CONSTRAINT "customer_name_uk" UNIQUE ("customer_name") 
  ... 
); 
CREATE TABLE "department" ( 
  "department_id" SERIAL NOT NULL, 
  "code" INT NOT NULL, 
  ... 
  CONSTRAINT "department_pk" PRIMARY KEY ("department_id"), 
  CONSTRAINT "department_code_uk" UNIQUE ("code&quot...