How Can I Efficiently Count Distinct Value Combinations Across Multiple Columns?

Distinct Counting Over Multiple Columns

Counting distinct values over multiple columns can be achieved through various methods. One approach involves utilizing a subquery, as showcased in the provided example:

SELECT COUNT(*) 
FROM (SELECT DISTINCT DocumentId, DocumentSessionId
      FROM DocumentOutputItems) AS internalQuery

This query leverages a subquery to obtain a unique combination of DocumentId and DocumentSessionId, then counts the number of distinct records in the resulting set.

Optimizing Performance

While the subquery approach may suffice for small datasets, larger datasets may warrant a more efficient solution. One optimization technique involves creating a persisted computed column based on a hash or concatenation of the two columns in question. This computed column becomes both indexable and statistically analyzable. By performing a distinct count on the computed column, you can achieve the same result as with the subquery, but with potentially improved performance due to the available optimization mechanisms.

The above is the detailed content of How Can I Efficiently Count Distinct Value Combinations Across Multiple Columns?. For more information, please follow other related articles on the PHP Chinese website!