Question

What is true about Unique and primary key?

(a) Unique can have multiple NULL values but Primary can’t have.

(b) Unique can have single NULL value but Primary can’t have even single.

(c) Both can have duplicate values

(d) None of the Mentioned

The question was posed to me in homework.

My doubt stems from Foreign, Candidate, Primary, Composite, Unique Keys topic in section Planning Oracle Applications of Oracle

Answer 1

Right option is (b) Unique can have single NULL value but Primary can’t have even single.

To explain I would say: Primary key doesn’t allow Null values and Unique key allows Null value, but only one Null value.