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Top 100 Amazon SQL Interview Questions and Answers

Top 100 Amazon SQL Interview Questions and Answers

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1. What is SQL and why is it important in the context of databases?

Answer:

SQL (Structured Query Language) is a programming language used for managing and querying relational databases. It’s important as it provides a standardized way to interact with databases, allowing users to retrieve, manipulate, and manage data efficiently.

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2. Explain the difference between SQL and NoSQL databases.

Answer:

SQL databases are relational and use a structured schema. They are suitable for complex query-intensive operations. NoSQL databases are non-relational, offering flexibility and scalability, making them ideal for large-scale, dynamic applications.

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3. Write an SQL query to retrieve all records from a table named ‘customers’.

Answer:

SELECT * FROM customers;

This query retrieves all rows and columns from the ‘customers’ table.

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4. What is the purpose of Amazon S3 (Simple Storage Service) in the AWS ecosystem?

Answer:

Amazon S3 is an object storage service that offers industry-leading scalability, data availability, security, and performance. It is designed to store and retrieve any amount of data from anywhere on the web.

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5. How do you use the DISTINCT keyword in an SQL query?

Answer:

The DISTINCT keyword is used to retrieve unique records from a column. For example:

SELECT DISTINCT column_name FROM table_name;

This query will return only unique values from the specified column.

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6. What is a JOIN in SQL, and what are the different types of joins?

Answer:

A JOIN combines rows from two or more tables based on a related column. Types of joins include INNER JOIN, LEFT JOIN (or LEFT OUTER JOIN), RIGHT JOIN (or RIGHT OUTER JOIN), and FULL JOIN (or FULL OUTER JOIN).

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7. Write an SQL query to find the second highest salary from an ’employees’ table.

Answer:

SELECT MAX(salary) 
FROM employees 
WHERE salary < (SELECT MAX(salary) FROM employees);

This query finds the highest salary that is less than the maximum salary, giving the second highest salary.

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8. What is an index in a database, and why is it important?

Answer:

An index is a data structure that improves the speed of data retrieval operations on a table. It allows the database to find rows more quickly, enhancing query performance.

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9. Write an SQL query to count the number of rows in a table named ‘products’.

Answer:

SELECT COUNT(*) FROM products;

This query returns the total number of rows in the ‘products’ table.

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10. Explain the purpose of the GROUP BY clause in an SQL query.

Answer:

The GROUP BY clause is used to group rows that have the same values into summary rows, like “find the total sales by region.” It is often used with aggregate functions like SUM, COUNT, AVG, etc.

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11. What is a subquery in SQL, and when would you use it?

Answer:

A subquery is a query within another query. It’s used to retrieve data based on criteria derived from another query. Subqueries are used when you need to perform operations based on intermediate results.

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12. Write an SQL query to find the names of employees who have a salary greater than the average salary.

Answer:

SELECT name 
FROM employees 
WHERE salary > (SELECT AVG(salary) FROM employees);

This query returns the names of employees whose salary is higher than the average salary.

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13. What is a self-join in SQL?

Answer:

A self-join is a specific type of join where a table is joined with itself. It’s useful for finding relationships within the same table, such as hierarchical data or comparing rows.

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14. Write an SQL query to find the third highest salary from an ’employees’ table.

Answer:

SELECT salary 
FROM employees 
ORDER BY salary DESC 
OFFSET 2 
LIMIT 1;

This query returns the third highest salary from the ’employees’ table.

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15. Explain the purpose of the HAVING clause in an SQL query.

Answer:

The HAVING clause is used in combination with GROUP BY to filter records based on a condition. It operates on aggregated data and is applied after the grouping.

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16. What is a trigger in SQL, and when would you use it?

Answer:

A trigger is a special type of stored procedure that is executed in response to a particular event on a table. It’s used for tasks like validation, auditing, or enforcing business rules.

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17. Write an SQL query to calculate the total sales for each product category.

Answer:

SELECT category, SUM(sales) 
FROM products 
GROUP BY category;

This query provides the total sales for each product category.

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18. What is a view in SQL, and why would you use it?

Answer:

A view is a virtual table that presents data from one or more tables. It doesn’t store data itself but provides a way to represent the data in a specific format. Views are used for security, simplicity, or to provide a different perspective on data.

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19. Write an SQL query to find the names of employees who do not have a manager.

Answer:

SELECT name 
FROM employees 
WHERE manager_id IS NULL;

This query returns the names of employees without a manager.

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20. Explain the purpose of the UNION and UNION ALL operators in SQL.

Answer:

The UNION operator is used to combine the result sets of two or more SELECT queries, removing duplicate rows. UNION ALL includes all rows, even if they are duplicates.

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21. What is a stored procedure in SQL, and why would you use it?

Answer:

A stored procedure is a set of SQL statements that can be stored in the database and executed later. It helps in modularizing code, improving performance, and providing a level of security.

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22. Write an SQL query to find the top 5 highest earning employees.

Answer:

SELECT name, salary 
FROM employees 
ORDER BY salary DESC 
LIMIT 5;

This query returns the names and salaries of the top 5 highest earning employees.

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23. Explain the purpose of the CASE statement in SQL.

Answer:

The CASE statement is used to perform conditional logic within an SQL query. It allows you to perform different actions based on different conditions.

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24. What is the difference between a primary key and a unique key in SQL?

Answer:

A primary key is a column (or combination of columns) that uniquely identifies each row in a table. It enforces entity integrity. A unique key ensures that values in a column (or combination) are unique but allows null values.

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25. Write an SQL query to find the names of employees who joined in the last month.

Answer:

SELECT name 
FROM employees 
WHERE EXTRACT(MONTH FROM hire_date) = EXTRACT(MONTH FROM CURRENT_DATE)
  AND EXTRACT(YEAR FROM hire_date) = EXTRACT(YEAR FROM CURRENT_DATE);

This query retrieves the names of employees who joined in the current month.

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26. What is normalization in database design?

Answer:

Normalization is the process of organizing data in a database to minimize redundancy and dependency. It involves dividing a database into two or more tables and defining relationships between them.

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27. Write an SQL query to find the average salary for each department.

Answer:

SELECT department, AVG(salary) 
FROM employees 
GROUP BY department;

This query calculates the average salary for each department.

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28. What is a foreign key in SQL?

Answer:

A foreign key is a field (or combination of fields) in one table that refers to the primary key in another table. It establishes a relationship between the two tables.

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29. Explain the concept of ACID properties in the context of databases.

Answer:

ACID (Atomicity, Consistency, Isolation, Durability) properties are a set of characteristics that ensure the reliability of transactions in a database system. They guarantee that database transactions are processed reliably.

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30. Write an SQL query to find the names of employees who have the same salary.

Answer:

SELECT name 
FROM employees 
WHERE salary IN (
  SELECT salary 
  FROM employees 
  GROUP BY salary 
  HAVING COUNT(*) > 1
);

This query retrieves the names of employees who share the same salary.

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31. What is the purpose of the TRUNCATE TABLE statement in SQL?

Answer:

The TRUNCATE TABLE statement is used to quickly remove all rows from a table. Unlike DELETE, it does not log individual row deletions, making it faster for large tables.

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32. Write an SQL query to find the total number of employees in each department.

Answer:

SELECT department, COUNT(*) 
FROM employees 
GROUP BY department;

This query provides the total number of employees in each department.

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33. Explain the purpose of the OFFSET and FETCH clauses in an SQL query.

Answer:

The OFFSET clause skips a specified number of rows in the result set, while FETCH limits the number of rows returned. They are often used together for pagination.

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34. Write an SQL query to find the highest salary for each department.

Answer:

SELECT department, MAX(salary) 
FROM employees 
GROUP BY department;

This query retrieves the highest salary for each department.

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35. What is a common table expression (CTE) in SQL?

Answer:

A CTE is a temporary result set that can be referenced within a SELECT, INSERT, UPDATE, or DELETE statement. It simplifies complex queries and makes them more readable.

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36. What is the difference between a clustered index and a non-clustered index in SQL?

Answer:

A clustered index determines the physical order of data in a table and is created on the actual data rows. It’s like sorting a phone book by the names. A non-clustered index is a separate structure that contains a sorted list of references to the table’s rows. It’s like an index at the end of a book.

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37. Write an SQL query to find the names of employees who earn more than the average salary.

Answer:

SELECT name 
FROM employees 
WHERE salary > (SELECT AVG(salary) FROM employees);

This query retrieves the names of employees earning more than the average salary.

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38. Explain the purpose of the EXISTS keyword in an SQL query.

Answer:

The EXISTS keyword is used to check if a subquery returns any results. It returns TRUE if the subquery returns one or more rows, otherwise FALSE.

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39. What is the purpose of the UNIQUE constraint in SQL?

Answer:

The UNIQUE constraint ensures that all values in a column (or combination of columns) are distinct. It prevents duplicate values from being inserted into the table.

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40. Write an SQL query to calculate the difference in days between two dates.

Answer:

SELECT DATEDIFF(day, '2023-09-13', '2023-09-30');

This query calculates the difference in days between September 30, 2023, and September 13, 2023.

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41. Explain the purpose of the ROLLUP operator in SQL.

Answer:

The ROLLUP operator is used in conjunction with the GROUP BY clause to generate subtotals and grand totals in result sets. It creates additional summary rows.

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42. Write an SQL query to find the top 10% of highest earning employees.

Answer:

SELECT name, salary 
FROM employees 
ORDER BY salary DESC 
LIMIT (SELECT COUNT(*) / 10 FROM employees);

This query returns the names and salaries of the top 10% highest earning employees.

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43. What is a self-join in SQL?

Answer:

A self-join is a join where a table is joined with itself. It is used to combine rows from the same table based on a related column between them.

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44. Write an SQL query to find the second highest salary in a table.

Answer:

SELECT MAX(salary) 
FROM employees 
WHERE salary < (SELECT MAX(salary) FROM employees);

This query retrieves the second highest salary from the employees table.

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45. Explain the purpose of the HAVING clause in SQL.

Answer:

The HAVING clause is used in combination with the GROUP BY clause to filter the result set based on a condition applied to aggregated values.

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46. What is the purpose of the COALESCE function in SQL?

Answer:

The COALESCE function returns the first non-null value from a list of values. It is used to handle NULL values in a more controlled manner.

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47. Write an SQL query to find the names of employees who do not belong to any department.

Answer:

SELECT name 
FROM employees 
WHERE department_id IS NULL;

This query retrieves the names of employees without a specified department.

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48. Explain the purpose of the UNION and UNION ALL operators in SQL.

Answer:

UNION combines the result sets of two or more SELECT queries into a distinct result set. UNION ALL also combines result sets, but allows duplicate rows.

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49. Write an SQL query to find the average salary for each department, including departments with no employees.

Answer:

SELECT d.department_name, AVG(e.salary) 
FROM departments d 
LEFT JOIN employees e ON d.department_id = e.department_id 
GROUP BY d.department_name;

This query calculates the average salary for each department, including those without employees.

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50. Explain the purpose of the CASE statement in SQL.

Answer:

The CASE statement is used to perform conditional logic in SQL queries. It allows you to return different values based on specified conditions.

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51. Write an SQL query to find the nth highest salary in a table.

Answer:

SELECT salary 
FROM employees 
ORDER BY salary DESC 
LIMIT 1 OFFSET n-1;

Replace ‘n’ with the desired rank. This query retrieves the nth highest salary from the employees table.

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52. Explain the purpose of the MERGE statement in SQL.

Answer:

The MERGE statement is used to perform insert, update, and delete operations in a single statement based on a specified condition.

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53. Write an SQL query to find the employees who have the same job title and department.

Answer:

SELECT a.name, a.job_title, a.department 
FROM employees a, employees b 
WHERE a.employee_id <> b.employee_id 
AND a.job_title = b.job_title 
AND a.department = b.department;

This query retrieves employees with the same job title and department.

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54. What is a correlated subquery in SQL?

Answer:

A correlated subquery is a subquery that depends on the outer query. It references columns from the outer query, allowing it to be executed once for each row processed by the outer query.

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55. Write an SQL query to find the departments with more than five employees.

Answer:

SELECT department, COUNT(*) 
FROM employees 
GROUP BY department 
HAVING COUNT(*) > 5;

This query retrieves departments with more than five employees.

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56. Explain the purpose of the LEAD and LAG functions in SQL.

Answer:

The LEAD and LAG functions provide access to subsequent and preceding rows in a result set. They are useful for comparing data across rows.

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57. What is a recursive common table expression (CTE) in SQL?

Answer:

A recursive CTE is a CTE that references itself. It’s used to perform recursive operations, like traversing hierarchical data structures.

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58. Write an SQL query to find the names of employees who have been with the company for more than five years.

Answer:

SELECT name 
FROM employees 
WHERE JOINING_DATE < DATEADD(year, -5, GETDATE());

This query retrieves employees who have been with the company for more than five years.

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59. Explain the purpose of the PIVOT and UNPIVOT operators in SQL.

Answer:

PIVOT is used to rotate rows into columns, aggregating data in the process. UNPIVOT does the opposite, converting columns into rows.

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60. Write an SQL query to find the employees who have not been assigned any projects.

Answer:

SELECT name 
FROM employees 
WHERE employee_id NOT IN (SELECT DISTINCT employee_id FROM projects);

This query retrieves employees without any assigned projects.

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61. What is the purpose of the ROW_NUMBER() function in SQL?

Answer:

The ROW_NUMBER() function assigns a unique number to each row in the result set. It is often used for pagination or ranking purposes.

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62. Write an SQL query to find the total salary expenditure for each department.

Answer:

SELECT department, SUM(salary) 
FROM employees 
GROUP BY department;

This query calculates the total salary expenditure for each department.

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63. Explain the purpose of the LEAST and GREATEST functions in SQL.

Answer:

LEAST returns the smallest value from a list of expressions, while GREATEST returns the largest value.

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64. What is the purpose of the HDFS (Hadoop Distributed File System) in Big Data?

Answer:

HDFS is a distributed file system that stores data across multiple machines. It provides high availability, fault tolerance, and scalability, making it a fundamental component of the Hadoop ecosystem.

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65. Explain the concept of MapReduce in the context of Big Data processing.

Answer:

MapReduce is a programming model for processing and generating large data sets in parallel across a distributed cluster. It consists of two main steps: Map, where data is divided into smaller chunks, and Reduce, where results are combined.

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66. What is the role of YARN (Yet Another Resource Negotiator) in Hadoop?

Answer:

YARN is the resource management layer of Hadoop. It manages and schedules resources across various applications in a Hadoop cluster, allowing for efficient utilization of resources.

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67. Explain the purpose of Hive in the Hadoop ecosystem.

Answer:

Hive is a data warehouse infrastructure that provides data summarization, query, and analysis capabilities. It uses a language similar to SQL, called HiveQL, to process data stored in HDFS.

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68. What is Spark in the context of Big Data processing?

Answer:

Apache Spark is an open-source, distributed computing system that provides fast data processing capabilities. It supports a wide range of applications, including batch processing, real-time streaming, and machine learning.

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69. Explain the purpose of Pig in the Hadoop ecosystem.

Answer:

Pig is a platform for analyzing large data sets. It provides a high-level scripting language, Pig Latin, which is used to express data analysis programs.

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70. What is the significance of Apache Zookeeper in distributed systems?

Answer:

Zookeeper is a coordination service that provides distributed synchronization and configuration management. It ensures that a distributed system behaves reliably and consistently.

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71. What is the purpose of HBase in the Hadoop ecosystem?

Answer:

HBase is a distributed, scalable, and consistent NoSQL database that runs on top of HDFS. It provides real-time read/write access to large datasets, making it suitable for applications with high-speed requirements.

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72. Explain the concept of data partitioning in a distributed database system.

Answer:

Data partitioning involves dividing a large dataset into smaller, more manageable parts. Each part, known as a partition, is stored on separate nodes in a distributed system, allowing for parallel processing.

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73. What is the purpose of Apache Kafka in a streaming data pipeline?

Answer:

Apache Kafka is a distributed event streaming platform that allows for the ingestion, processing, and storage of large volumes of streaming data in real-time.

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74. Explain the concept of data skew in a distributed computing environment.

Answer:

Data skew occurs when the distribution of data across nodes in a cluster is uneven. This can lead to performance issues, as some nodes may become overloaded while others remain underutilized.

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75. What is the purpose of Amazon Kinesis in the AWS ecosystem?

Answer:

Amazon Kinesis is a fully managed service for real-time streaming data processing. It allows for the collection, processing, and analysis of large streams of data.

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76. Explain the role of Amazon Redshift in data warehousing.

Answer:

Amazon Redshift is a fully managed data warehousing service that allows for the efficient storage and querying of large datasets. It is designed for analytics and business intelligence applications.

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77. What is the purpose of AWS Glue in the AWS ecosystem?

Answer:

AWS Glue is a fully managed extract, transform, and load (ETL) service that makes it easy for users to prepare and load their data for analytics.

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78. What is the purpose of Amazon EMR (Elastic MapReduce) in the AWS ecosystem?

Answer:

Amazon EMR is a cloud-based big data platform that simplifies the processing of vast amounts of data using popular open-source frameworks like Apache Spark and Hadoop.

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79. Explain the concept of data lineage in a data processing pipeline.

Answer:

Data lineage refers to the path that data takes from its source to its destination, including all the transformations and processes it undergoes along the way. It helps in tracking the origin and transformation of data.

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80. What is the purpose of Amazon Athena in the AWS ecosystem?

Answer:

Amazon Athena is an interactive query service that allows you to analyze data stored in Amazon S3 using standard SQL. It eliminates the need for complex ETL processes.

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81. Explain the concept of data serialization in the context of distributed computing.

Answer:

Data serialization is the process of converting complex data structures or objects into a format that can be easily stored, transmitted, and reconstructed later. It is crucial in distributed systems for efficient data exchange.

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82. What is the purpose of Amazon QuickSight in the AWS ecosystem?

Answer:

Amazon QuickSight is a cloud-based business analytics service that enables users to visualize and analyze data quickly. It integrates with various data sources and provides interactive dashboards.

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83. Explain the concept of eventual consistency in distributed databases.

Answer:

Eventual consistency is a consistency model in which a system guarantees that, given enough time, all replicas of a piece of data will converge to the same value, even in the presence of concurrent updates.

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84. What is the purpose of AWS Data Pipeline in the AWS ecosystem?

Answer:

AWS Data Pipeline is a web service for orchestrating and automating the movement and transformation of data between different AWS services and on-premises data sources.

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85. What is the purpose of Amazon Quicksight in the AWS ecosystem?

Answer:

Amazon QuickSight is a business analytics tool that enables users to create and share interactive dashboards and visualizations. It can connect to various data sources, making it a powerful tool for data analysis.

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86. Explain the concept of sharding in database management.

Answer:

Sharding involves breaking up a large database into smaller, more manageable pieces called shards. Each shard is stored on a separate server, allowing for improved performance and scalability.

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87. What is the purpose of Amazon Neptune in the AWS ecosystem?

Answer:

Amazon Neptune is a fully managed graph database service that allows you to build and run applications that work with highly connected datasets. It supports both property graph and RDF graph models.

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88. Explain the concept of data compression in the context of big data storage.

Answer:

Data compression involves reducing the size of data to save storage space and improve processing speed. It works by encoding information using fewer bits than the original representation.

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89. What is the purpose of Amazon Redshift Spectrum in the AWS ecosystem?

Answer:

Amazon Redshift Spectrum extends the querying capabilities of Amazon Redshift to analyze data stored in Amazon S3. It allows for querying large amounts of data without the need to load it into a Redshift cluster.

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90. Explain the concept of columnar storage in data warehouses.

Answer:

Columnar storage organizes data by columns rather than by rows. This allows for faster queries on specific columns, making it highly efficient for analytics and reporting.

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91. What is the purpose of AWS Glue DataBrew in the AWS ecosystem?

Answer:

AWS Glue DataBrew is a visual data preparation tool that makes it easy for users to clean and normalize data for analytics and machine learning.

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92. What is the purpose of Amazon MSK (Managed Streaming for Apache Kafka) in the AWS ecosystem?

Answer:

Amazon MSK is a fully managed service that makes it easy to build and run applications that use Apache Kafka for streaming data processing.

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93. Explain the concept of ETL (Extract, Transform, Load) in data processing.

Answer:

ETL is a process used in data integration where data is extracted from one source, transformed into a format suitable for analysis, and loaded into a target system, typically a data warehouse.

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94. What is the purpose of AWS Lake Formation in the AWS ecosystem?

Answer:

AWS Lake Formation is a service that makes it easy to set up, secure, and manage a data lake. It simplifies the process of building, securing, and managing a data lake.

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95. Explain the concept of CAP theorem in distributed systems.

Answer:

The CAP theorem states that it is impossible for a distributed system to simultaneously achieve Consistency, Availability, and Partition tolerance. In any distributed system, you can only have two out of the three.

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96. What is the purpose of Amazon S3 Select in the AWS ecosystem?

Answer:

Amazon S3 Select allows you to retrieve only a subset of data from an object in Amazon S3, which can significantly improve query performance and reduce the amount of data transferred.

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97. Explain the concept of data skew in the context of distributed computing.

Answer:

Data skew occurs when the distribution of data across nodes in a cluster is uneven. This can lead to performance issues, as some nodes may become overloaded while others remain underutilized.

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98. What is the purpose of Amazon RDS (Relational Database Service) in the AWS ecosystem?

Answer:

Amazon RDS is a managed relational database service that makes it easier to set up, operate, and scale a relational database in the cloud.

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99. What is the purpose of Amazon Kinesis in the AWS ecosystem?

Answer:

Amazon Kinesis is a platform for streaming data on AWS. It allows you to easily collect, process, and analyze real-time, streaming data so you can get timely insights and react quickly to new information.

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100. Explain the concept of data skew in the context of distributed computing.

Answer:

Data skew occurs when the distribution of data across nodes in a cluster is uneven. This can lead to performance issues, as some nodes may become overloaded while others remain underutilized.

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