Part $1$

$(9$ points$)$ Please give a $$True$$ or a $$False$$ rating to each statement below.

a$.$ A schema that is in Third Normal Form $(3$NF$)$ must also be in Second Normal Form $(2$NF$).$

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b$.$ A foreign key can contain a NULL.

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c$.$ The union rule allows combining functional dependencies A$$CD and A$$EF into a functional dependency A$$CDEF$.$

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d$.$ SQL query output is always sorted by the primary key.

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e$.$ A column that appears in the WHERE clause must always appear in the SELECT clause.

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f$.$ A UNIQUE constraint on a column allows insertion of a NULL.

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