Cardinality is ?

• A. The number of rows
• B. The number of distinct values in a table column relative to the number of rows
• C. The average length of values
• D. The maximum value in the column

Answer: (B)

Cardinality is about how many distinct values exist in a column, and it’s usually described relative to the total number of rows. The best description here says it’s the number of distinct values in a table column relative to the number of rows. This matters because it affects how selective a predicate on that column will be during query processing—a column with many unique values (high cardinality) can quickly narrow results, while a column with few unique values (low cardinality) may not filter as much.

The other options miss what cardinality means: the total number of rows isn’t about distinct values, the average length of values isn’t about how many unique values there are, and the maximum value isn’t about the count of distinct values. For example, a column with many unique IDs across a large table has high cardinality, whereas a boolean flag or a status column that only takes a few values has low cardinality.