SQL Interview Questions and Answers- Part 2

LISTEN TO THE SQL FAQs LIKE AN AUDIOBOOK

SQL Interview Questions and Answers- Part 2

SQL is the backbone of data handling in most companies. If you’re aiming for top-paying roles like Data Analyst, Database Administrator, or BI Developer, SQL will almost always be part of the interview process. Interviewers use SQL questions to check how you work with real-world data—whether it’s retrieving information, filtering results, or joining multiple tables.

This set of SQL interview questions and answers covers a wide range of topics like subqueries, aggregate functions, indexing, normalization, and optimization. Each question is designed to help you think logically and demonstrate your problem-solving skills with data.

Practicing essential SQL interview questions builds confidence. You’ll encounter queries on joins, group by clauses, window functions, and indexing strategies. These questions test your ability to handle relational databases efficiently. While confidence is important, employers are ultimately looking for depth of knowledge and the ability to apply SQL in complex, real-world scenarios.

Today’s roles are a mixture of data analyst, data scientist, data engineer, and AI specialist. That means SQL alone isn’t enough. Employers expect candidates to master visualization tools like Power BI and Tableau, statistical platforms like SAS, and cloud services such as Azure. Data engineering expertise in Snowflake, Databricks, and PySpark is critical, as is proficiency in machine learning and AI frameworks like TensorFlow, PyTorch, LLMs, and Generative AI. Project work showcasing these skills is often the deciding factor in hiring decisions.

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Answer:

UNION and UNION ALL are used to combine the result sets of two or more SELECT statements.

  • UNION: Combines the result sets and removes duplicate rows. It performs a distinct operation.
  • UNION ALL: Combines the result sets without removing duplicate rows. It does not perform a distinct operation.

Answer:

  • INNER JOIN: Returns only the matching rows between two tables based on the join condition.
  • OUTER JOIN: Returns all the rows from one table and the matching rows from the other table. If there are no matching rows, NULL values are returned for the columns of the other table.

Answer:

Clustered index: It determines the physical order of data rows in a table. There can be only one clustered index per table as it defines the table’s physical structure.

Non-clustered index: It is a separate structure that contains a sorted list of the indexed column’s values and a pointer to the actual data rows. A table can have multiple non-clustered indexes.

Answer:

To install SQL, you need to follow these general steps:

  1. Choose a Database Management System (DBMS): SQL is a language used to interact with relational databases, so you first need to choose a DBMS that supports SQL. Some popular options include MySQL, PostgreSQL, Microsoft SQL Server, Oracle Database, and SQLite. Each DBMS has its own installation process.
  2. Download the DBMS: Visit the official website of the chosen DBMS and navigate to the downloads section. Look for the appropriate version of the DBMS for your operating system.
  3. Install the DBMS: Once the download is complete, run the installer and follow the installation wizard. You may need to choose the installation directory and configure some settings along the way. The exact steps will depend on the DBMS you’re installing.
  4. Start the DBMS: After the installation is complete, start the DBMS service. This is typically done through the services or applications menu of your operating system. Some DBMS may have a specific program to launch.
  5. Verify the installation: Open a command prompt or terminal and type the appropriate command to check if the DBMS is installed correctly. For example, for MySQL, you can type mysql –versionto see the installed version.

Answer:

When it comes to SQL Server, it is not possible to rename the column with the help of ALTER TABLE command, you can use the sp_rename system stored procedure.

Answer:

There are several types of SQL queries that are commonly used to interact with a database. Here are some of the most common types:

  1. SELECT: The SELECT query is used to retrieve data from one or more tables in a database
  2. INSERT: The INSERT query is used to insert new data into a table.
  3. UPDATE: The UPDATE query is used to modify existing data in a table.
  4. DELETE: The DELETE query is used to delete one or more rows from a table.
  5. JOIN: The JOIN query is used to combine rows from two or more tables based on a related column between them.
  6. CREATE: The CREATE query is used to create new database objects such as tables, views, indexes, or stored procedures.
  7. ALTER: The ALTER query is used to modify the structure of an existing database object.
  8. DROP: The DROP query is used to delete an existing database object such as a table, view, index, or stored procedure.

Answer:

Nested triggers refer to a feature in Microsoft SQL Server that allows the firing of additional triggers when a trigger is already being executed. In SQL Server, triggers are special stored procedures that are automatically executed in response to specific events, such as an INSERT, UPDATE, or DELETE operation on a table.

Answer:

There are three levels of nested triggers:

  1. Level 0: Nested triggers are not allowed. When a trigger is already executing, any additional triggers that would be fired as a result of the current trigger are not executed.
  2. Level 1: Nested triggers are allowed, but only one level deep. When a trigger is already executing, additional triggers that would be fired as a result of the current trigger are executed. However, if those triggers also fire additional triggers, those nested triggers are not executed.
  3. Level 2: Nested triggers are allowed, and multiple levels of nesting are permitted. When a trigger is already executing, additional triggers that would be fired as a result of the current trigger are executed, even if they themselves fire additional triggers.

The level of nested triggers is configured using the sp_configure system stored procedure. Changing the nested triggers setting should be done with caution because it can have performance implications and increase the risk of trigger-related issues like infinite loops.

Answer:

In SQL, OLTP stands for “Online Transaction Processing.” It refers to a system or approach designed for managing and processing transactions in real-time. OLTP systems are commonly used in database management and are optimized for handling a large number of small, individual transactions.

Answer:

The primary goal of OLTP systems is to ensure data integrity and consistency while allowing multiple users to perform transactions concurrently. These systems often involve frequent reads and writes to the database and focus on maintaining high availability and fast response times.

Answer:

OLAP stands for Online Analytical Processing. It is a category of software tools and technologies used for data analysis and reporting. OLAP allows users to perform complex and multidimensional analysis of large datasets, often referred to as data cubes.

Answer:

Here are some key characteristics of OLAP:

  1. Multidimensional Analysis: OLAP allows users to analyze data from multiple dimensions or perspectives. Dimensions can include various attributes or parameters, such as time, geography, product, customer, and so on. This multidimensional view helps users gain insights and make informed decisions.
  2. Aggregation: OLAP supports the aggregation of data across different dimensions. It allows users to summarize data at various levels of detail, such as total sales per month, sales by region, or sales by product category. Aggregated data can be quickly accessed and analyzed, enabling faster decision-making.
  3. Fast Query Performance: OLAP systems are optimized for fast query performance. They use specialized data structures and indexing techniques to efficiently retrieve and process data, even when dealing with large datasets. This allows users to interactively explore data and receive quick responses to their queries.
  4. Drill-Down and Roll-Up: OLAP provides the capability to drill down into detailed data or roll up to higher-level summaries. Users can navigate through different levels of data granularity to explore the underlying details or analyze data at a higher level of abstraction.
  5. Slice-and-Dice: OLAP allows users to slice and dice data, which means filtering or selecting specific subsets of data based on certain criteria. Users can focus on particular dimensions or combinations of dimensions to analyze specific aspects of the data.
  6. Advanced Calculations: OLAP tools often provide built-in support for advanced calculations and formulas. Users can create calculated measures, perform mathematical operations, define custom calculations, and apply various functions to the data for in-depth analysis.

Answer:

In SQL, the alternative to the TOP clause depends on the specific database management system (DBMS) you are using. Here are some alternatives for the TOP clause in popular DBMS:

  • ROWCOUNT
  • Set rowcount 3
  • Select * from employee order by empid desc Set rowcount 0

Answer:

There are five types of sorting algorithms which are as follows:

  • Selection Sort
  • Bubble Sort
  • Bucket Sort
  • Quick Sort
  • Merge Sort

Answer:

The SQL HAVING clause and the WHERE clause are both used in SQL queries, but they serve different purposes.

  • The WHERE clause is used to filter rows based on specific conditions. It is used in the SELECT, UPDATE, DELETE, and INSERT The WHERE clause is applied before any grouping or aggregation takes place. It determines which rows are included in the result set.
  • On the other hand, the HAVING clause is used to filter groups in SQL queries that involve grouping and aggregation, such as GROUP BY It is applied after the GROUP BY clause and any aggregation functions like COUNT, SUM, AVG, etc. The HAVING clause allows you to specify conditions that must be met by the grouped rows.

Answer:

To disable a trigger in SQL, you can use the DISABLE TRIGGER statement. The specific syntax may vary depending on the database management system (DBMS) you are using. Ensure to replace “trigger_name” with the actual name of the trigger you want to disable and “table_name” with the name of the table on which the trigger is defined.

Answer:

During a database white box testing process, the following activities may be performed:

  1. Database Structure Analysis: Testers analyze the design of the database, including the relationships between tables, indexes, and constraints. They ensure that the structure is efficient and follows the defined business rules.
  2. SQL Code Review: Testers review the SQL queries, stored procedures, triggers, and other database code. They check for correctness, performance issues, and potential security vulnerabilities.
  3. Code Coverage Analysis: Testers assess the code coverage by determining which parts of the database code have been executed during testing. This helps ensure that all code paths have been tested and that there are no untested or unreachable portions.
  4. Performance Testing: Testers evaluate the performance of the database system under different workloads and stress conditions. They may examine the query execution plans, indexing strategies, and overall system response times.
  5. Security Testing: Testers verify the security measures implemented in the database system, such as user access controls, encryption, and data integrity mechanisms. They aim to identify any potential vulnerabilities that could be exploited by malicious users.

Answer:

A subquery, also known as a nested query or inner query, is a query that is embedded within another query. It is used in SQL (Structured Query Language) to retrieve data from one or more tables based on the results of another query.

A subquery is enclosed within parentheses and typically placed within the WHERE clause or HAVING clause of the outer query. The result of the subquery is used by the outer query to perform further filtering or comparison operations.

Answer:

To create a stored procedure using SQL Server, you can follow these steps:

  1. Open SQL Server Management Studio (SSMS) or any other SQL Server query tool.
  2. Connect to your SQL Server instance.
  3. Open a new query window or select an existing database.
  4. Write the SQL code for creating the stored procedure
  1. Write the SQL statements and logic within the BEGINand END These statements will be executed when the stored procedure is called.
  2. Optionally, you can add comments to describe the purpose of the stored procedure and any specific logic.
  3. Execute the SQL code by clicking the “Execute” button or pressing the F5 key.

Answer:

SQL (Structured Query Language) and NoSQL (Not only SQL) are two different types of database management systems (DBMS) with distinct characteristics. Here are the key differences between them based on different factors:

  1. Data Model: SQL databases follow a relational data model, where data is organized into tables with predefined schemas. Whereas, NoSQL databases have a flexible data model that can vary across different records.
  2. Scalability: SQL databases traditionally have vertical scalability, meaning they can handle increased data loads by upgrading hardware resources. On the contrary, NoSQL databases excel in horizontal scalability. They are designed to distribute data across multiple servers or clusters, allowing for seamless scaling by adding more machines to the system.
  3. Data Integrity and Consistency: SQL databases enforce ACID (Atomicity, Consistency, Isolation, Durability) properties, ensuring data consistency and integrity. In contrast, NoSQL databases typically prioritize scalability and availability over strict consistency. They often follow the BASE (Basically Available, Soft state, Eventual consistency) principle, where data consistency is eventually achieved after a period of time.
  4. Query Language: SQL databases use SQL as the standard language for defining and manipulating data. However, NoSQL databases have varying query languages based on their data model.
  5. Use Cases: SQL databases are well-suited for structured data with complex relationships, such as financial systems, e-commerce platforms, and applications requiring complex querying and reporting. NoSQL databases excel in handling large volumes of unstructured or semi-structured data, making them suitable for applications like content management systems, real-time analytics, and social media platforms.