Practice mode DBMS and SQL

Relation R(A, B, C) has A as its only candidate key. How many superkeys does R have?

Correct answer B. 4

Explanation

Every superset of a candidate key is a superkey. With A fixed, each of B and C can be either included or left out, giving 2 x 2 = 4 superkeys: {A}, {A,B}, {A,C} and {A,B,C}. The answer 8 counts every subset of three attributes, including the empty set and sets without A, which cannot identify tuples. In general, with one single-attribute key among n attributes there are 2^(n-1) superkeys.

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Relation R(A, B, C) has A as its only candidate key. How many superkeys does R have?

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Relation R(A, B, C) has A as its only candidate key. How many superkeys does R have?

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