SQL Interview Questions 2026

A production table was accidentally deleted 2 hours ago. How would you recover the data using Snowflake Time Travel?

A critical table was dropped 10 days ago and the Time Travel retention period has expired. What recovery options are available? Explain how Fail-safe works and its limitations.

Your team needs a test environment containing production data without duplicating storage costs. How would you use Zero Copy Cloning and what are its benefits and limitations?

Snowpipe is ingesting duplicate records from cloud storage. How would you detect, prevent, and remediate duplicate data in Snowflake?

A query that previously completed in 2 minutes now takes 20 minutes. Describe your systematic approach to troubleshooting and optimizing performance in Snowflake.

Compare Snowpipe and COPY INTO. When would you choose one over the other?

Compare Time Travel and Fail-safe in Snowflake. What are the key differences and use cases for each?

Compare Temporary, Transient, and Permanent tables in Snowflake. When would each table type be appropriate?

Compare Streams and Tasks in Snowflake. How do they work together to support Change Data Capture (CDC) workflows?

Your Snowflake query is slow even with a large warehouse. How do you investigate and optimize it?

You have a 10TB Snowflake table queried daily by 50 analysts with very different filter patterns. How do you optimize it?

What is Time Travel in Snowflake and how does it differ from Fail-Safe?

Write a query to find customers who have placed orders in every month of the current year, given an Orders table with customer_id and order_date.

Design the booking system for a home-rental marketplace, ensuring two guests can never double-book the same property for overlapping dates under concurrent requests.

Write a query to find hosts whose average review rating dropped by more than 0.5 stars comparing their most recent 10 reviews to their prior 10 reviews.

Explain idempotency keys in a payments API. What specific failure mode do they prevent, and how would you design the storage layer for them so they don't become a bottleneck?

Design a double-entry ledger system for tracking money movement between accounts. Why is this pattern preferred over simply updating a balance column, especially for auditability?

Write a query to find the top 3 skills most commonly shared between pairs of connected users, given a UserSkills table and a Connections table.

Design a multi-tenant database architecture for a CRM SaaS product with 100,000 customers, where each customer's data must be strictly isolated but the schema is largely shared. Compare separate databases per tenant vs. shared schema with a tenant_id column.

Write a query to find customers who placed an order in each of the last 3 consecutive months, using an Orders table with customer_id and order_date.

Design the data model for a Kanban board (like Trello/Jira) supporting boards, columns, and cards, where cards can be reordered within and across columns efficiently without renumbering every card on each move.

A query that joins 4 tables and runs fine on a 1M-row test database takes 40 seconds in production on 500M rows. Walk through your approach to diagnosing and fixing it.

Explain the difference between INNER JOIN, LEFT JOIN, and FULL OUTER JOIN with a simple example of Orders and Customers tables.

A SELECT query filtering on a non-indexed column takes 8 seconds on a 10 million row table. Would adding an index always help? What's the trade-off of adding too many indexes?

What problem does database normalization (1NF, 2NF, 3NF) actually solve? Give an example of an update anomaly in an unnormalized table.

Write a query to find, for each department, the employee with the second-highest salary, using window functions instead of a correlated subquery.

How would you find out why a specific SQL query is slow before making any changes to it?

Write a query to find, for each city, the percentage of rides that were cancelled within 2 minutes of being booked, given a Rides table with city, booked_at, and cancelled_at.