SQL Blog

Commonly Asked SQL Questions

Dr. Cecelia Allison

What are Common Careers in Data and the Role Structured Query Language (SQL) Plays in Each?
(Train for a Career in Data.)

Following are common careers in data and the role of Structured Query Language (SQL):

1. Data Analyst:

Role: Data analysts interpret and analyze data to uncover trends, patterns, and insights that can guide business decisions.

SQL Role: Data analysts use SQL to retrieve specific data subsets from databases like Microsoft Access. SQL helps them filter, aggregate, and transform data for analysis, and create custom reports.

2. Data Scientist:

Role: Data scientists work with complex data to develop predictive models, machine learning algorithms, and statistical analyses to extract valuable insights.

SQL Role: SQL is used by data scientists to gather and preprocess data for analysis. It aids in merging datasets, handling missing values, and structuring data for machine learning tasks.

3. Business Analyst:

Role: Business analysts focus on understanding business needs, processes, and requirements to optimize operations and facilitate decision-making.

SQL Role: Business analysts use SQL to query databases for information needed to assess performance, create ad hoc reports, and support strategic planning.

4. Market Researcher:

Role: Market researchers gather, analyze, and interpret data about markets, customers, and competitors to provide insights for marketing strategies.

SQL Role: SQL is used to extract relevant data from databases for analysis. It helps market researchers identify trends, consumer behavior, and preferences.

5. Quality Assurance/Software Tester:

Role: QA testers ensure software quality by identifying and fixing defects, validating functionality, and ensuring a positive user experience.

SQL Role: SQL is used to verify data integrity, test database interactions, and validate data-related aspects of applications, ensuring accurate information storage and retrieval.

6. Database Administrator:

Role: Database administrators manage, maintain, and secure databases, ensuring data availability, integrity, and performance.

SQL Role: Database administrators use SQL to create, modify, and optimize database structures, manage user permissions, and fine-tune queries for efficient data retrieval.

7. Data Modeler:

Role: Data modelers design the logical and physical structure of databases, including tables, relationships, and constraints.

SQL Role: SQL is employed to create and modify database objects, define data types, establish relationships, and enforce data integrity rules.

In summary, SQL plays a central role in each of these job roles by enabling professionals to access, manipulate, and analyze data stored in databases. It supports tasks ranging from basic data retrieval and filtering to complex analysis, modeling, and maintenance activities, depending on the specific needs of each role.

How to Quickly Learn SQL & Begin a Career Working with Data? (Get an SQL Certificate!)

1. WATCH & Subscribe to the SQL Tutorial on YouTube and learn SQL FREE with Hands-On Practice in SQL Server.

SUBSCRIBE to Receive Notifications on New SQL Content and to Keep Your SQL Skills Sharp.

2. WATCH Playlist: Learn How SQL Professionals Land SQL Jobs.

Use the Playlist above to discover how SQL professionals began their careers as:

  • Data Analysts

  • Data Engineers

  • Data Scientists

  • Business Analysts

  • Quality Assurance Software Testers

  • Cybersecurity Architects

  • Product Managers

  • Marketing Analysts

  • SQL Developers

  • Full Stack Developers

  • 3. Research SQL Jobs on indeed.com


    4. UpGrade Your Professional Skills

    Access Top-Rated Udemy Learning Resources Below:

  • Artificial Intelligence (AI) & ChatGPT

  • Cyber Security

  • Cloud Computing

  • Microsoft Excel for Business

  • Relational Database Design

  • Microsoft SQL Server

  • Python

  • 5. Check out Popular SQL Resources on Amazon

    SQL Resources & Books!

    6. Earn a SQL Certification

    SQL Certificates

    7. Earn a Microsoft Credential

    Microsoft Certifications

    How Popular is Structured Query Language (SQL)? (Get an SQL Certificate!)

    SQL (Structured Query Language) remains highly popular and widely used in the field of database management. SQL is the standard language for interacting with relational database management systems (RDBMS), and it plays a crucial role in managing and manipulating data in databases.

    SQL's popularity can be attributed to several factors:

    Ubiquity: SQL is supported by virtually all major relational database systems, including MySQL, PostgreSQL, Oracle, Microsoft SQL Server, and SQLite. This widespread support makes it a go-to choice for working with databases.

    Standardization: SQL is an industry-standard language, which means that the basic syntax and functionality remain consistent across different database systems. This standardization enhances portability and ease of learning.

    Versatility: SQL is versatile and can be used for a wide range of tasks, including data querying, data manipulation, database design, and data definition. It provides a comprehensive set of commands for managing relational databases.

    Structured Querying: The structured nature of SQL queries makes it easier for users to retrieve and manipulate data in a structured format. This is especially important for businesses and organizations dealing with large volumes of structured data.

    Integration with Applications: Many programming languages and applications support SQL, enabling seamless integration between databases and other software systems.

    Maturity: SQL has been around for several decades, and its maturity as a language contributes to its stability and reliability.

    Given these factors, it's likely that SQL's popularity has continued to endure.

    What are Common SQL Interview Questions? (Get an SQL Certificate!)

    Following are common SQL interview questions.

    SQL interview questions can vary depending on the specific role and the level of expertise required. However, here are some common

    SQL interview questions that are often asked across different job roles:

    1. Basic SQL Questions:

    What is SQL, and what does it stand for?

    Explain the differences between SQL and NoSQL databases.

    What are the basic components of an SQL query?

    How do you comment in SQL?

    2. Querying and Retrieving Data:

    How do you retrieve all records from a table?

    Explain the difference between the SELECT and FROM clauses.

    How do you filter data using the WHERE clause?

    What is the ORDER BY clause used for, and how do you use it?

    How can you limit the number of rows returned in a query?

    3. Joins and Relationships:

    What are different types of joins in SQL?

    Explain the differences between INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN.

    How do you handle NULL values when working with joins?

    4. Aggregation and Grouping:

    What are SQL aggregate functions (e.g., SUM, COUNT, AVG, MAX, MIN)?

    How do you group data using the GROUP BY clause?

    What is the HAVING clause, and when do you use it?

    5. Subqueries:

    What is a subquery, and how is it used in SQL?

    Provide an example of a subquery in a query.

    6. Data Modification:

    How do you insert new records into a table?

    How do you update existing records in a table?

    How do you delete records from a table?

    7. Indexes and Performance:

    What is an SQL index, and why is it important?

    How do indexes affect the performance of queries?

    8. Normalization:

    What is database normalization, and why is it essential?

    Explain the different normalization forms (e.g., 1NF, 2NF, 3NF).

    9. Transactions and ACID Properties:

    What is a database transaction?

    Explain the ACID properties in the context of database transactions.


  • What is an SQL view, and why do we use it?

  • How do you create and use views in SQL?

  • These are just a few examples of common SQL interview questions. Depending on the job role and the level of expertise required, interviewers might ask more complex questions or questions related to specific database management systems like MySQL, PostgreSQL, SQL Server, or Oracle. It's essential to be well-prepared and have a solid understanding of SQL concepts and practical applications to excel in SQL interviews.

    What are the Top In-Demand Skills to Become a Data Analyst? (Get an SQL Certificate!)

    Following are some of the top in-demand skills for becoming a data analyst.

    The field of technology and data analysis is constantly evolving, so it's a good idea to check for the most recent trends and updates:

    1. SQL (Structured Query Language): SQL is used for querying and manipulating relational databases. A strong understanding of SQL is essential for data retrieval and analysis from databases.

    2. Data Manipulation and Cleaning: Proficiency in data cleaning, transformation, and manipulation using tools like Python (pandas) or R is essential. Dealing with messy, incomplete, or inconsistent data is a significant part of a data analyst's role.

    3. Data Visualization: Being able to create clear and effective visualizations of data is important for communicating insights to non-technical stakeholders. Tools like Tableau, Power BI, matplotlib, or seaborn in Python are commonly used.

    4. Statistical Analysis and Mathematics: A solid foundation in statistics and mathematics is essential for understanding and interpreting data. Skills in probability, hypothesis testing, regression analysis, and other statistical techniques are crucial.

    5. Programming Languages: Proficiency in programming languages like Python or R is crucial for data analysis. These languages are used for data manipulation, analysis, and automation of tasks.

    6. Machine Learning Basics: Understanding the fundamentals of machine learning is valuable for more advanced data analysis tasks. Knowledge of algorithms, model evaluation, and feature engineering can be beneficial.

    7. Domain Knowledge: Depending on the industry you're working in, having domain-specific knowledge can provide context and help you ask the right questions when analyzing data.

    8. Big Data Tools: Familiarity with big data tools and technologies like Hadoop, Spark, or Hive can be advantageous when dealing with large datasets.

    9. Excel Skills: While not as advanced as some other tools, Excel is still widely used in data analysis for tasks like basic calculations, data organization, and simple visualization.

    10. Communication Skills: The ability to effectively communicate your findings to both technical and non-technical audiences is crucial. Clear writing and presentation skills are important to convey insights and recommendations.

    11. Version Control/Git: Proficiency in version control systems like Git can help you collaborate with team members, track changes, and manage your analysis code efficiently.

    12. Critical Thinking and Problem-Solving: Data analysts often need to approach complex problems with a critical and analytical mindset to derive meaningful insights.

    13. Data Ethics and Privacy: Understanding the ethical considerations and privacy concerns related to handling and analyzing data is becoming increasingly important.

    Remember that the specific skills in demand might vary based on the industry, company, and the evolving landscape of data analysis. To stay current, consider networking with professionals in the field, taking online courses or certifications, and staying updated on industry trends through blogs, forums, and conferences.

    What jobs rely heavily on SQL skills? (Get an SQL Certificate!)

    Following are jobs that rely heavily on SQL skills.

    There are several jobs that require mainly SQL (Structured Query Language) skills.

    Many businesses and organizations rely on databases to store, retrieve, and manage their data, which is why there is a consistent demand for professionals with strong SQL skills. Here are some job roles that heavily involve SQL:

    1. Database Administrator (DBA): DBAs are responsible for maintaining, configuring, and ensuring the integrity of databases. They handle tasks such as data backup and recovery, performance tuning, security management, and database design. Proficiency in SQL is essential for these tasks.

    2. Data Analyst: Data analysts use SQL to query databases, extract relevant data, and perform data analysis to provide insights to support business decisions. They work with data to identify trends, patterns, and correlations.

    3. Business Intelligence (BI) Developer: BI developers create tools, reports, and dashboards that help organizations visualize and understand their data. SQL skills are crucial for extracting data and building the necessary queries for these tools.

    4. Data Engineer: Data engineers design, build, and manage the infrastructure required for data generation, storage, and retrieval. SQL is used to handle data pipelines, ETL (Extract, Transform, Load) processes, and data integration.

    5. Database Developer: Database developers design, implement, and optimize databases. They create the tables, relationships, and queries necessary to store and retrieve data efficiently.

    6. ETL Developer: ETL developers specialize in building Extract, Transform, Load processes, which involve extracting data from various sources, transforming it into a suitable format, and loading it into a data warehouse or database.

    7. Quality Assurance (QA) Tester: QA testers often use SQL to validate the accuracy and integrity of data within applications or databases. They write queries to verify that data is stored, retrieved, and processed correctly.

    8. Data Scientist (to some extent): While data scientists primarily work with various programming languages and tools, SQL can still be valuable for retrieving and preprocessing data from databases before conducting analysis.

    9. Financial Analyst: Financial analysts may use SQL to access financial databases, retrieve relevant financial data, and create reports for analysis and decision-making.

    10. Healthcare Informatics Specialist: In healthcare settings, professionals may use SQL to manage patient data, generate reports, and ensure compliance with data security regulations.

    11. IT Support Analyst: IT support professionals may use SQL to troubleshoot database-related issues, perform diagnostics, and ensure proper functioning of applications that rely on databases.

    These roles often require a combination of technical skills beyond just SQL, such as knowledge of specific database management systems (e.g., MySQL, PostgreSQL, SQL Server), scripting languages, and data analysis tools. However, strong SQL skills are a fundamental requirement for success in these positions.

    Is it Worth Getting a SQL Certification? (Get an SQL Certificate!)

    Obtaining a SQL certification can be a valuable investment for your career, but its worth depends on various factors, including your career goals, current expertise, and the industry in which you work or plan to work.

    Here are some points to consider when deciding whether to pursue a SQL certification:

    Career Advancement: If you are looking to advance your career in fields like database administration, data analysis, data engineering, or software development, a SQL certification can enhance your credibility and increase your chances of landing better job opportunities.

    Skill Validation: A SQL certification demonstrates that you possess a certain level of expertise in SQL, which can be appealing to employers and clients who are seeking skilled professionals.

    Competitive Edge: In competitive job markets, a certification can differentiate you from other candidates, giving you an advantage in the hiring process.

    Learning and Knowledge: Even if you have experience with SQL, pursuing a certification can deepen your knowledge and understanding of the language, potentially exposing you to advanced concepts and best practices.

    Industry Recognition: Certain certifications are well-known and respected in the industry, which can add credibility to your resume.

    Employer Requirements: Some employers specifically look for candidates with SQL certifications, especially when hiring for roles heavily focused on database management and data analysis.

    On the other hand, there are some aspects to consider that might impact the decision:

    Cost: Certification exams often come with a cost, and some may require training or study materials, which can add up.

    Time Commitment: Preparing for a certification exam takes time and effort, which could be a concern if you have other pressing commitments.

    Experience vs. Certification: In some cases, relevant work experience may be more valuable than a certification. If you already have a strong track record and practical experience with SQL, the certification might not be as critical.

    Evolution of Technology: The value of certifications may vary over time as technology evolves. Make sure to research the relevance and recognition of the specific certification you are considering.

    In conclusion, while a SQL certification can be beneficial and bolster your career prospects, it is essential to assess your individual circumstances, career goals, and the specific certification program's reputation before deciding whether it's worth pursuing. Remember that certifications should complement your skills and experience, not be a substitute for them.

    What are common SQL questions? (Get an SQL Certificate!)

    Common SQL questions cover a wide range of topics and difficulty levels.

    Here are some additional common SQL interview questions, expanding on the previous list:

    1. SQL Constraints:

    Explain various types of SQL constraints (e.g., NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, CHECK).

    How do you add and remove constraints in an existing table?

    2. SQL Functions:

    What are SQL scalar functions, and how are they used?

    Provide examples of SQL aggregate functions and their purposes.

    3. SQL Subqueries:

    What is the difference between correlated and non-correlated subqueries?

    How do you use subqueries to solve complex problems?

    4. SQL Common Table Expressions (CTEs):

    What are CTEs, and why would you use them?

    How do you create and use CTEs in SQL queries?

    5. SQL Self-Joins:

    Explain the concept of a self-join and when it is applicable.

    Provide an example of a self-join in a real-world scenario.

    6. SQL Window Functions:

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

    How do you use window functions to perform calculations across a result set?

    7. SQL Transactions and Concurrency:

    Explain the concept of a database transaction and why it is essential.

    How do you ensure data consistency and handle concurrency issues in SQL?

    8. SQL Index Optimization:

    How do you optimize database performance using indexes?

    Discuss scenarios where adding or removing indexes can be beneficial.

    9. SQL Stored Procedures and Functions:

    What are SQL stored procedures and functions?

    How do they differ, and how do you create and execute them?

    10. Normalization and Denormalization:

    Describe the process of database normalization.

    When might you denormalize a database, and what are the trade-offs?

    11. SQL Injection:

    What is SQL injection, and how can it be prevented?

    12. Handling Date and Time in SQL:

    How do you work with date and time data types in SQL?

    How can you extract specific components from date and time values?

    13. Database Design:

    Describe the process of designing a relational database schema.

    Explain the concept of database normalization and its advantages.

    Remember that the complexity of SQL questions may vary depending on the level of the SQL role you are interviewing for, so it's essential to be well-prepared and have a solid understanding of SQL fundamentals and best practices. Practical hands-on experience with SQL queries and databases can greatly enhance your ability to answer these questions effectively during an interview.

    What is the Difference Between a Left Outer Join and a Right Outer Join in SQL? (Get an SQL Certificate!)

    The outer join returns all rows from one table and only those rows from a secondary table where the joined fields are equal (join condition is met).

    The following query displays EVERY record from the Customer table and those records in the Orders table that have a corresponding Customer ID in the Customer table:

    SELECT Customer.CustomerID, Orders.PlanID

    FROM Customer LEFT OUTER JOIN Orders

    ON Customer.CustomerID = Orders.CustomerID;

    The LEFT OUTER JOIN keywords tell the DBMS to include every row in the table (Customer) to the left of the LEFT OUTER JOIN keywords. The ON keyword is used to specify the condition (Customer.CustomerID = Orders.CustomerID).

    In the results from the query, every Customer ID from the Customer table is retrieved. Even those Customers that have not ordered any items yet.

    Keep in mind, that a right outer join and a left outer join is basically the same thing. It just depends on how you set up the query.

    For example, both of the following queries are equivalent:

    SELECT Customer.CustomerID, Orders.PlanID

    FROM Customer LEFT OUTER JOIN Orders

    ON Customer.CustomerID = Orders.CustomerID;

    SELECT Customer.CustomerID, Orders.PlanID

    FROM Orders RIGHT OUTER JOIN Customer

    ON Customer.CustomerID = Orders.CustomerID;

    How to Copy Table Data for One Table to Another Table in SQL Server? (Get an SQL Certificate!)

    To copy and paste data from one table to another in SQL you can use 2 methods.

    Method 1. Copy Data to a New Table

    Following is the syntax to copy data to a new table:

    SELECT ColumnNames

    INTO NewTable

    FROM CurrentTable;

    For example the following query copies records from an existing Customer table and creates a new Customer table (NewCustomer) and then copies the records into the new Customer table (NewCustomer):

    SELECT *

    INTO NewCustomer

    FROM Customer;

    Method 2. Copy Data to an Existing Table

    Following is the syntax to copy data to an existing table:

    INSERT INTO ExistingTable (ColumnNames)

    SELECT ColumnNames

    FROM CurrentTable;

    For Example the following query copies data from the existing Customer table to an existing table named Customer2:

    INSERT INTO Customer2 (CustomerID, FirstName, LastName, HomePhone, Address, State, City, PostalCode)

    SELECT CustomerID, FirstName, LastName, HomePhone, Address, State, City, PostalCode

    FROM Customer;