Data & SQL · INTERVIEW & OA PREP

Topic 8 of 12

SQL for Data Roles

For data analyst, data science, and many backend roles, a live SQL round is standard. Joins, aggregation, and window functions cover the vast majority of what's asked — practice writing them without autocomplete.

Key concepts

• JOIN types: inner, left, right, full — and when each changes the result.
• GROUP BY with HAVING to filter aggregates (vs WHERE for rows).
• Window functions: ROW_NUMBER, RANK, running totals with OVER().
• Subqueries and CTEs for readable multi-step logic.

Questions that get asked

• Find the second-highest salary.
• Count of employees per department, only where count > 5.
• Nth highest value using a window function.
• Find duplicate rows in a table.
• Join orders to customers and show customers with no orders.