Here are 50 interview questions related to Advanced SQL Topics:

Window Functions:

1. What are window functions in SQL, and how do they differ from standard aggregate functions?

Answer: Window functions perform calculations across a set of table rows related to the current row, without reducing the result set. They differ from aggregate functions, which reduce rows to a single result.

2. Explain the purpose of the ROW_NUMBER() window function in SQL and provide an example.

Answer: ROW_NUMBER() assigns a unique integer to each row within a result set. For example, it can be used to assign a rank to each product in a product list.

3. What is the difference between RANK() and DENSE_RANK() window functions, and when would you use each?

Answer: RANK() assigns the same rank to rows with the same values, leaving gaps. DENSE_RANK() assigns the same rank to rows with the same values but without gaps. Use RANK() when you want to indicate gaps, and DENSE_RANK() when you want to avoid them.

4. Explain the purpose of the PARTITION BY clause in window functions, and how is it used?

Answer: PARTITION BY divides the result set into partitions, and window functions are applied within each partition. It is used to perform calculations on a per-partition basis.

5. What is the LEAD() and LAG() window function, and how are they used for retrieving data from neighboring rows?

Answer: LEAD() and LAG() allow you to access data from the next or previous row in the result set, respectively. They are used to retrieve data from neighboring rows for comparison or calculation.

6. Explain the concept of the NTILE() window function and how it can be used to divide data into equal-sized partitions.

Answer: NTILE() divides the result set into the specified number of partitions, ensuring that each partition has a roughly equal number of rows. It’s useful for creating percentile groups.

7. What is the purpose of the FIRST_VALUE() and LAST_VALUE() window functions, and how are they used?

Answer: FIRST_VALUE() returns the value of an expression from the first row in a window frame, and LAST_VALUE() returns the value from the last row in a window frame. They are used to retrieve boundary values within a window.

8. Explain the use of the ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW clause in window functions.

Answer: The clause specifies the frame in which a window function operates, including all rows from the start of the partition to the current row. It is used to perform calculations on all preceding rows.

9. What is the significance of the ROWS BETWEEN n PRECEDING AND m FOLLOWING clause in window functions?

Answer: The clause specifies the frame for a window function, including “n” preceding rows and “m” following rows relative to the current row. It allows you to define a specific range for calculations.

10. Explain how you can use window functions to calculate running totals or cumulative sums.

Answer: Running totals are calculated using the SUM() window function with the appropriate frame specification to accumulate values up to the current row.

Common Table Expressions (CTEs):

11. What is a Common Table Expression (CTE) in SQL, and how is it used to simplify complex queries?

Answer: A CTE is a named temporary result set that can be referenced within a SQL query. It simplifies complex queries by breaking them down into more manageable parts.

12. What is the difference between a recursive CTE and a non-recursive CTE, and when would you use each?

Answer: A non-recursive CTE is used for straightforward queries, while a recursive CTE is used to work with hierarchical or recursive data, such as organizational charts or nested categories.

13. Explain the structure of a recursive CTE, including the anchor member and recursive member.

Answer: A recursive CTE includes an anchor member, which defines the initial results, and a recursive member, which generates subsequent rows by referring to the CTE itself.

14. What is the MAXRECURSION option in a recursive CTE, and how is it used to limit recursion?

Answer: MAXRECURSION is an option that limits the number of recursion levels in a recursive CTE. It is used to prevent infinite loops and control the depth of recursion.

15. What is the significance of the UNION operator when used within a recursive CTE, and how does it affect the result set?

Answer: The UNION operator combines the results of the anchor and recursive members in a recursive CTE. It’s used to form the final result set by merging initial and subsequent data.

16. Explain the concept of memoization in the context of recursive CTEs, and how does it improve query performance?

Answer: Memoization is a caching technique in recursive CTEs, where results are stored and reused to reduce redundant calculations, improving query performance.

17. How do you reference a CTE within a SQL query, and what are the benefits of using CTEs for readability and maintenance?

Answer: A CTE is referenced using its defined name within the main query. CTEs improve query readability and maintenance by breaking complex queries into modular, named parts.

18. What is the WITH RECURSIVE keyword, and how is it used to define a recursive CTE in SQL?

Answer: The WITH RECURSIVE keyword is used to declare a CTE as recursive. It allows the CTE to refer to itself within the recursive member.

19. Explain the concept of cascading CTEs, and how can they be used to build complex queries step by step?

Answer: Cascading CTEs are a series of CTEs, where each CTE builds upon the previous one. They can be used to break down a complex query into multiple, more manageable steps.

20. What is the significance of the common table expression’s scope, and how is it determined within a query?

Answer: The scope of a CTE is limited to the query where it’s defined. It is determined by the location of the CTE within the query.

Advanced Join Techniques:

21. What is a self-join in SQL, and how is it used to join a table with itself?

Answer: A self-join is a join operation where a table is joined with itself. It’s often used to compare rows within the same table, such as finding employees who manage other employees.

22. Explain the concept of a cross join (Cartesian product) in SQL and provide an example of when it might be used.

Answer: A cross join combines all rows from one table with all rows from another table, creating a Cartesian product. It is used to generate all possible combinations, such as matching students with courses to explore enrollment options.

23. What is a FULL OUTER JOIN in SQL, and how does it differ from INNER JOIN and LEFT JOIN?

Answer: A FULL OUTER JOIN returns all rows from both tables, matching rows from the left and right tables where possible and including non-matching rows from both. It differs from INNER JOIN, which returns only matching rows, and LEFT JOIN, which returns all rows from the left table and matching rows from the right.

24. Explain the concept of a semi-join and anti-join in SQL, and how are they used to retrieve specific subsets of data?

Answer: A semi-join returns rows from the left table where there is a match in the right table. An anti-join returns rows from the left table where there is no match in the right table. They are used to filter data based on specific conditions.

25. What is a self-exclusion join, and how can it be used to remove rows from a table that match certain criteria within the same table?

Answer: A self-exclusion join, or self-anti-join, is used to remove rows from a table that match certain criteria within the same table. It is used when you want to exclude specific rows based on internal conditions.

26. Explain the purpose of the CROSS APPLY and OUTER APPLY operators in SQL, and how are they used in conjunction with joins?

Answer: CROSS APPLY and OUTER APPLY are used to apply a table-valued function to each row of another table. They can be used to perform complex calculations and transformations during a join operation.

27. What is a PIVOT operation in SQL, and how does it relate to advanced joins and data transformation?

Answer: A PIVOT operation is used to transform row data into columnar data, effectively “rotating” data for easier analysis. It is related to advanced joins as it often involves aggregating and grouping data before pivoting.

28. Explain the concept of a correlated subquery within a join operation, and when is it useful for data retrieval?

Answer: A correlated subquery in a join operation uses values from the outer query to filter results from the subquery. It is useful when you need to retrieve data based on context-dependent conditions.

29. What is the purpose of a lateral join in SQL, and how is it used for subqueries within a join operation?

Answer: A lateral join, often used with the LATERAL keyword, allows subqueries to reference columns from the outer query. It is used to perform complex calculations within a join operation.

30. Explain the concept of a NATURAL JOIN in SQL and the challenges it can pose in practice.

Answer: A NATURAL JOIN is used to join tables based on columns with the same name. It can be challenging in practice because it relies on column naming conventions and can lead to unexpected results if columns change.

Window Functions:

31. What is the purpose of the PERCENT_RANK() and CUME_DIST() window functions, and how are they used to calculate percentiles?

Answer: PERCENT_RANK() returns the relative rank of a row within a result set, and CUME_DIST() returns the cumulative distribution. They are used to calculate percentiles and cumulative distribution values for a set of data.

32. Explain the OVER() clause in SQL window functions, and how is it used to define the window frame for calculations?

Answer: The OVER() clause specifies the window frame for window functions, including the PARTITION BY clause and the frame specification. It is used to define the subset of rows over which calculations are performed.

33. What are the limitations of window functions in older SQL database systems, and how have these limitations been addressed in newer versions?

Answer: Older SQL database systems may have limitations in terms of support for window functions. Newer versions have improved support, adding new functions and capabilities.

34. Explain the concept of a moving average calculation using window functions, and how can it be used for time-series data analysis?

Answer: A moving average is calculated using a window frame that moves through a time-series data set. It is used to smooth data and identify trends over time.

35. What is the DENSE_RANK() function, and how is it different from the RANK() function in SQL window functions?

Answer: DENSE_RANK() assigns the same rank to rows with the same values without gaps, similar to RANK(). The key difference is that DENSE_RANK() does not leave gaps when multiple rows share the same rank.

Common Table Expressions (CTEs):

36. What are recursive CTEs, and how do they enable querying hierarchical data, such as organizational structures or category trees?

Answer: Recursive CTEs allow for self-referencing queries, making them suitable for hierarchical data like organizational charts or category hierarchies. They help navigate parent-child relationships.

37. What is the primary use case for non-recursive CTEs, and how do they improve query readability and maintenance?

Answer: Non-recursive CTEs are typically used to simplify complex queries by breaking them down into modular, named parts. They enhance query readability and maintenance by providing a structured approach.

38. Explain the limitations of recursive CTEs, and how can you address performance issues with large hierarchical data?

Answer: Recursive CTEs may have performance limitations with large data sets. To address this, you can use indexing, materialized paths, or alternative methods like nested set models.

39. What is the difference between a temporary table and a CTE in SQL, and when might you choose one over the other?

Answer: A temporary table is a physical table, while a CTE is a temporary result set. You might choose one over the other based on the query’s complexity and the need for temporary storage.

40. Explain the role of the RECURSIVE keyword when defining a recursive CTE, and why is it necessary?

Answer: The RECURSIVE keyword is used to specify that a CTE is recursive. It’s necessary to indicate that the CTE will refer to itself in the recursive member.

Advanced Join Techniques:

41. What is a self-union join in SQL, and how is it used to combine data within the same table?

Answer: A self-union join combines data from the same table by using table aliases to treat the table as two separate entities. It’s useful for joining records with similar characteristics.

42. Explain the concept of a recursive self-join and its role in querying hierarchical data structures.

Answer: A recursive self-join is used to navigate hierarchical data structures. It joins a table to itself to establish parent-child relationships, making it suitable for tasks like traversing organization charts.

43. What is the significance of a full outer join in SQL, and when might you use it to retrieve data from two tables?

Answer: A full outer join returns all rows from both tables, including matches and non-matches. It’s used when you want to combine data from two tables while preserving all records.

44. What is the difference between an anti-join and a semi-join in SQL, and how can they be used for data retrieval and filtering?

Answer: An anti-join returns rows from the left table that have no matching rows in the right table, while a semi-join returns rows from the left table where there is a match in the right table. They are used for data filtering based on specific conditions.

45. Explain the concept of a self-exclusion join, and provide an example of when it might be useful in a query.

Answer: A self-exclusion join, or self-anti-join, is used to exclude rows from a table that match certain criteria within the same table. It can be useful when you need to eliminate specific rows based on internal conditions.

46. What is the CROSS APPLY operator in SQL, and how does it differ from other join types like INNER JOIN?

Answer: CROSS APPLY is used to apply a table-valued function to each row of another table. It differs from INNER JOIN by executing a function for each row, potentially returning multiple rows per row in the main table.

47. Explain the concept of a PIVOT operation in SQL and how it’s used for data transformation and summarization.

Answer: A PIVOT operation is used to transform row data into columnar data for easier analysis and summarization. It is employed for tasks like generating pivot tables and cross-tabulations.

48. What is the role of a lateral join in SQL, and how does it affect the behavior of subqueries within a query?

Answer: A lateral join, often used with the LATERAL keyword, allows subqueries to reference columns from the outer query. It is used to perform complex calculations and transformations within a join operation.

49. Explain the concept of a natural join in SQL and the challenges it can pose when working with databases.

Answer: A natural join joins tables based on columns with the same name. Challenges include reliance on naming conventions, the potential for unexpected results, and the need for strict column naming consistency.

50. What are the considerations when using the EXCEPT and INTERSECT set operators in SQL for data comparison and retrieval?

Answer: EXCEPT returns distinct rows from the left table that are not in the right table, while INTERSECT returns distinct rows present in both tables. Considerations include column compatibility, data type matching, and the desired outcome for set operations.