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

Counting Unique Values Across Multiple Columns: Alternatives to Subqueries

When faced with the task of counting unique values from a table based on multiple columns, such as DocumentId and DocumentSessionId, the natural inclination is to employ a subquery approach. However, this may raise concerns about performance optimization.

One alternative to using a subquery involves creating a persisted computed column. This can be achieved by hashing or concatenating the desired columns, ensuring a deterministic result that can be indexed and have statistics built on it.

For instance, if the columns in question are DocumentId and DocumentSessionId, a computed column named HashValue could be created using a hash function such as SHA1 or MD5:

ALTER TABLE DocumentOutputItems ADD COLUMN HashValue AS SHA1(DocumentId + DocumentSessionId);

Once the computed column is persisted and indexed, a distinct count on this column will be equivalent to the desired result:

SELECT COUNT(DISTINCT HashValue) FROM DocumentOutputItems;

By using a persisted computed column, you can leverage indexing and statistics to improve performance, eliminating the need for a subquery.

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