Câu 87: DA0-001: Data+

Which of the following is a difference between a primary key and a unique key?

1. A. A unique key cannot take null values, whereas a primary key can take null values.
2. B. There can be only one primary key in a data set, whereas there can be multiple unique keys. — đáp án hiện tại
3. C. A primary key can take a value more than once, whereas a unique key cannot take a value more than once.
4. D. A primary key cannot be a date variable, whereas a unique key can be.