I just rawdogged the A/B testing based on my prior knowledge from college lol. I focused more on the SQL part. I think what they want to see the most is how familiar are you with data processing and how you deal with data related issues. The simplest example is: I always keep in mind what is the granularity of the source data and the datamart that I want to make. Usually I check the source first (count per id, count per policy number, etc, depending on the data)