Write a query to find the second highest salary from the Employee table. If there is no second highest salary, return NULL.
This classic interview question tests your understanding of aggregate functions and subqueries. Airbnb frequently uses this to assess candidates' ability to handle ranking and filtering operations, which are common in their pricing and availability systems. The challenge lies in handling edge cases: what if there's only one unique salary? What if all salaries are the same? Your solution must gracefully return NULL in these scenarios.
This problem primarily tests: MAX() aggregate function, nested subqueries for comparison, handling NULL values when no second highest exists, and understanding DISTINCT for duplicate handling. Alternative approaches include ORDER BY with LIMIT/OFFSET or window functions like DENSE_RANK().
In production systems, similar queries are used for: finding the second-best pricing option for dynamic pricing algorithms, identifying runner-up candidates in recommendation systems, analyzing performance metrics where you need to exclude the top outlier, and calculating fallback values in availability searches when the primary option is unavailable.
When tackling this Airbnb problem, the key is to understand the grain of the result. Are you returning one row per user, or one row per category? Always start by identifying your unique join keys and consider if filtered aggregations (CASE WHEN) are more efficient than multiple subqueries.
Be careful with NULL values in your JOIN conditions or aggregate functions. In interview scenarios, datasets often include edge cases like zero-count categories or duplicate entries that can throw off a simple COUNT(*) if not handled with DISTINCT.
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