How Can I Efficiently Count Distinct Combinations Across Multiple Columns in a Database Table?

Counting Distinct Values Across Multiple Columns

To count the number of distinct items based on multiple columns in a table, consider the following query:

SELECT COUNT(DISTINCT DocumentId, DocumentSessionId)
FROM DocumentOutputItems

This query efficiently counts the distinct combinations of DocumentId and DocumentSessionId, providing the desired result.

Alternative Approaches

While the provided query offers a straightforward solution, alternative approaches may improve performance in certain scenarios:

Using Persisted Computed Columns:

For a better performance, a persisted computed column can be created based on a hash or concatenation of DocumentId and DocumentSessionId. This allows the database to index and create statistics on the computed column.

ALTER TABLE DocumentOutputItems
ADD ComputedColumn AS HASHBYTES('MD5', DocumentId + DocumentSessionId) PERSISTED

Once the computed column is populated, a distinct count on it will be equivalent to the original query:

SELECT COUNT(DISTINCT ComputedColumn)
FROM DocumentOutputItems

By optimizing the database structure and minimizing the number of queries, these approaches can enhance the performance of distinct counting operations across multiple columns.

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