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Consider the following three relations in a relational database. Employee(eId, Name), Brand(bId, bName), Own(eId, bId) Which of the following relational algebra expressions return the set of eIds who own all the brands?

1. Π_eId (Π_eId,bId (Own) / Π_bId (Brand))
2. Π_eId (Own) − Π_eId ((Π_eId (Own) × Π_bId (Brand)) − Π_eId,bId (Own))
3. Π_eId (Π_eId,bId (Own) / Π_bId (Own))
4. Π_eId ((Π_eId (Own) × Π_bId (Own)) / Π_bId (Brand))

Correct answer: Π_eId (Π_eId,bId (Own) / Π_bId (Brand))

Solution

This expression correctly identifies eIds that own all brands by performing a division operation, which effectively filters out eIds that do not have ownership of every brand listed in the Brand relation.

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