My Favorite SQL Interview Question

When it comes to SQL interview questions, there are a myriad of challenges that candidates might face. However, one of my favorite questions to pose involves a scenario that is both practical and insightful.

The Scenario

Imagine you have a table in a Snowflake database with the following columns:

• user_id an integer that uniquely identifies a user.
• movie_id an integer that uniquely identifies a movie.
• watch_time a timestamp indicating the time the user started watching the movie.

Your task is to find the most popular sequence of the first and second movies watched by users. This kind of query could provide valuable insights for a streaming service looking to understand viewing patterns and enhance recommendations.

The SQL Challenge

To solve this problem, we need to craft a SQL query that can:

1. Determine the order in which each user watched their movies.
2. Isolate the sequences where the first and second movies were watched.
3. Aggregate these sequences to find which pair is the most popular.

Here's how we can achieve this with a Snowflake SQL query:

Breaking Down the Query

The query is divided into parts using Common Table Expressions (CTEs) for clarity and structure:

1. Ranking Movies: The ranked_movies CTE is used to assign a rank to each movie watched by a user, ordered by the time they started watching it.
2. Pairing Sequences: The first_and_second_movies CTE then pairs the first and second ranked movies for each user. This is done by self-joining the ranked CTE on the user ID and ensuring the second movie's rank is immediately after the first.
3. Aggregating Results: Lastly, the query groups the results by the movie pair and counts the occurrences. The ORDER BY clause then ensures that the most common sequences are listed first, giving us the insight we need.

Why This Question?

This question is a gem for several reasons:

• Real-World Relevance It simulates an actual problem that data analysts and engineers might need to solve.
• Complexity It requires understanding of window functions, CTEs, joins, and group by clauses.
• Insightful It goes beyond the code; the answer can provide actionable insights for business decisions.

Conclusion

Not only does this SQL interview question test the candidate's technical prowess, but it also gauges their ability to think critically about data and its implications for user behavior analysis. It's a question that encapsulates the essence of what it means to work with data: it's not just about queries and tables, it's about the stories and patterns that emerge when you know how to ask the right questions.