SQL Interview Tips

Do not memorize syntax; memorize structures. Master the five patterns in Writing Queries and most data-logic questions reduce to one of them.

How Interviews Work

Round 1: Single Pattern Test

Interviewers start simple. The first question tests one pattern in isolation:

  • "Find the top 3 products by revenue" → Ladder pattern

  • "Show user activity over time" → Timeline pattern

  • "Find users who did X but not Y" → Funnel pattern

Round 2: Pattern Composition

In the second round, they ask you to combine patterns.

  • "Top 3 products by revenue over time" → Ladder + Timeline

  • "Users who played but never shared, then flag the sudden spikes" → Funnel + Exception

  • "Peak usage hours by user segment" → Timeline + Ladder

Why interviewers reward CTEs

Why interviewers like CTEs:

  • They show you can break a complex problem into modular steps.

  • Each CTE handles one step (for example, compute the average before you compute the offset).

  • They are straightforward to debug and to explain aloud.

Systems Design Bonus: Scaling Reads

If the interview pivots from writing queries to Systems Design ("How do we scale this database when traffic spikes?"), do not jump to a NoSQL migration.

Instead, name the primary-plus-read-replica pattern: standard SQL databases scale read-heavy workloads by deploying read replicas. Citing ChatGPT's ~800 million users on standard Postgres with read replicas shows you reach for the simplest scaling tool first.

Practice Strategy

  1. Start Single: Master individual patterns on your datasets.

  2. Combine: Progress to combining 2 patterns (e.g., Ladder + Funnel to map top conversion paths).

  3. Alias Smartly: In an interview, readability matters. Use user_signup_rank instead of generic aliases like r.