Some entities cannot be told apart by their own attributes. Two exams both have a "question 3":

question is a weak entity setweak entity setAn entity set with no key of its own; it is identified by the key of an owner entity set plus a partial key:

• it has no key of its own,
• it depends on an owner (identifying) entity set, here exam, through an identifying relationship,
• its partial key (discriminator), here question_no, tells apart the weak entities of one owner,
• its participation in the identifying relationship is total: every question belongs to an exam.

In an ER diagram, a weak entity set and its identifying relationship have double borders. The partial key has a dashed underline.

As a table

CREATE TABLE question (
  exam_id     integer REFERENCES exam (exam_id) ON DELETE CASCADE,
  question_no integer,
  body        text,
  points      integer,
  PRIMARY KEY (exam_id, question_no)
);

• Primary key: the owner's key plus the partial key.
• The owner's key is also a foreign key.
• ON DELETE CASCADE (Chapter 5): a weak entity cannot exist without its owner.