DDR Data Science Interview Practice Test

Which statement about a unique key is true?

A unique key ensures all values in a column are distinct and allows one NULL value. A unique key cannot have NULL values. A unique key identifies a row uniquely. A unique key functions identically to a primary key.

Correct Answer:

A unique key ensures all values in a column are distinct and allows one NULL value.

Explanation:

A unique key enforces that the values in its column (or set of columns) are distinct across all rows. This means a given non-NULL value can appear at most once, so you can identify a row by its unique value. It also allows NULL values; in practice, many databases permit multiple NULLs because NULL represents unknown and NULLs are not considered equal to each other. So the core idea is that a unique key guarantees uniqueness of values and permits NULLs, unlike a primary key which cannot be NULL.

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