# Advanced SQL for Data Engineering (Data Engineering)

> Window functions, recursive CTEs, analytical queries, optimization, EXPLAIN ANALYZE, indexing, partitioning

- 20 interview questions
- Mid-Level
- [Interview Questions: Data Engineering](https://sharpskill.dev/en/technologies/data-engineering/interview-questions.md)

## 1. Which window function assigns a unique sequential number to each row within a partition?

**Answer**

ROW_NUMBER() assigns a unique sequential number to each row within a partition, starting at 1. Unlike RANK() and DENSE_RANK() which can assign the same number to equal values, ROW_NUMBER() always guarantees unique numbers, even for identical values in the ORDER BY clause.

## 2. What is the main difference between RANK() and DENSE_RANK()?

**Answer**

RANK() leaves gaps in the numbering after tied values. For example, if two rows are ranked 1, the next one will be ranked 3. DENSE_RANK() doesn't leave gaps: after two rows ranked 1, the next will be ranked 2. This distinction is important for ranking analyses where gaps can skew statistics.

## 3. How to calculate a running total (cumulative sum) with a window function?

**Answer**

SUM() OVER with ORDER BY and no explicit frame uses ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW by default, which calculates the sum of all preceding rows up to and including the current row. This is the standard behavior for creating running totals in time-series or sequential analyses.

## 17 more questions available

- What is the role of the PARTITION BY clause in a window function?
- Which function allows accessing the value of the previous row in a window function?

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