When should you use SELECT DISTINCT queries?

• A. Only when duplicate data exists in multiple data extensions
• B. Always, to ensure uniqueness
• C. Never, as it slows queries
• D. Only when there is a single data extension

Answer: (A)

SELECT DISTINCT is about returning only unique rows in the query results. You’d use it when the data you’re pulling could produce duplicates, which often happens when you combine data from more than one data source or when joining data extensions. In these cases, without distinct, the same record could appear multiple times in the result, and DISTINCT removes those duplicates so each row appears once.

It isn’t something you apply all the time—if you know the data will be unique for the columns you’re selecting, or you actually want duplicates for counting, you’d skip it because it adds overhead. Also, duplicates can arise from multiple data extensions or sources, not just a single one, so the situation described in the option reflects a common reason to use DISTINCT.