Explain SQL Window Functions.

Example

A PostgreSQL query like:

SELECT date, sales, SUM(sales) OVER (ORDER BY date) AS running_total  
FROM daily_sales;

This shows each day’s sales with a running total across time.

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Why Is It Important

They let you perform advanced analytics in a single query, reducing the need for subqueries or multiple joins. This makes SQL both cleaner and faster to maintain.

Interview Tips

Explain clearly that window functions do not reduce row count like GROUP BY. Be ready to mention PARTITION BY and ORDER BY as the core syntax. Use ranking or running total examples to stand out.

Trade-offs

They simplify logic but can be expensive on very large datasets, requiring careful indexing.

Pitfalls

Common mistakes include forgetting PARTITION BY, misusing ORDER BY, or assuming results are aggregated like GROUP BY.