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My material on database claims that:

Weak entities help to avoid data duplication and inconsistencies arising due to duplicating the key of the strong entity.

My question is how can a strong entity key even be duplicated? This would make the key lose its uniqueness property.

Can anyone please help? How is the statement provided in the material justified?

Thanks in advance!

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Definition and example of weak entity

The diagram shows a strong entity (Order, key Order number) linked to multiple weak entities (Order Line, key Order number+Order linenumnber). There's a foreign key from the weak entity to the strong (Order number).

So the Order number is duplicated in the sense a given Order number appears in many entities: one strong (Order number is the whole key); many weak (Order number is part of the key).

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