Characterisitics of weak entities

Question

In E-R model , weak entity meets which of the following conditions

1. Weak entity existence dependent i.e. weak entity cannot exist without the entity with which it has a relationship
2. weak entity has a primary key that is partially or totally derived from the parent in the relationship
3. Both (1) and (2)
4. Neither 1 nor 2

Comments

Option 3. Both are correct.

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How can primary key of a weak entity be totally derived from parent / identifying entity? Can you explain what is meant by "totally derived from"? I felt, it means primary key of weak entity does not contain any attribute of weak entity but is made up solely of attributes from parent / identifying entity. Is it correct? If yes then how is possible? How a relation can have primary key fully made up of attributes from other relation? Can you give any example?

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Can You explain how?