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SQL Interview Questions

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  1. How do you explain what SQL is to someone without a technical background, and how do you decide which details to include? See answer
  2. What are the different types of keys in SQL and when do you use them? See answer
  3. How would you prepare an SQL database for migration to a new server or a cloud-based service? See answer
  4. What are the types of normalization available in SQL and how do you use them? See answer
  5. How would you reassure a stakeholder who’s concerned about a SQL database slowdown that’s getting in the way of their work? See answer
  6. Can you list all of the joins in SQL and the use case for each of them? See answer
  7. How would you explain the concept of a query to a non-technical audience, such as a client or end user? See answer
  8. What is the importance of data integrity in your work with SQL? See answer
  9. What are the different constraints used in SQL and how can they be applied?
  10. How would you improve data retrieval time without altering the current schema or the database?
  11. Are you comfortable with cluster environment maintenance and setup?
  12. What size databases have you worked with in the past and how would you approach working with a larger database?
  13. What steps would you take to troubleshoot connection issues when trying to access the database server?
  14. What communication strategies do you use to effectively collaborate with sysadmins to resolve code issues?
  15. Are you familiar with the normalization process for large data sets?
  16. Tell me about your methods for maintaining database security when making updates.
  17. How would you successfully lead a meeting with a range of remote developers and IT support technicians?
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8 SQL Interview Questions and Answers

Q:

How do you explain what SQL is to someone without a technical background, and how do you decide which details to include?

A:

SQL administrators and technicians may spend a lot of time interacting with end users and teams who rely heavily on the databases. The applicant should feel comfortable explaining details of SQL and the way it relates to the end user’s work.
What to look for in an answer:

  • Clear process on explaining SQL to non-technical staff
  • Deep understanding of SQL concepts
  • Explanation of how they choose the most important parts of SQL to include

Example:

“SQL stands for Structured Query Language. This language is how database administrators communicate with a database and manage its contents. For example, I use a particular string of commands and variables, called a query, to look up values in the database.”

Q:

What are the different types of keys in SQL and when do you use them?

A:

You can use this technical question to gauge whether an applicant has the right amount of SQL knowledge. Keys are a relatively basic concept in SQL, so anyone with experience in this technology should know what they are.
What to look for in an answer:

  • Strong understanding of what keys are and how they’re used in SQL
  • Skill at appropriately using keys in SQL
  • Ability to explain the keys concept in an interview environment

Example:

“A unique key is a unique field or column that a table doesn’t share with any other table in the SQL database. I use this key to create a unique identifier for each table for easier retrieval later.”

Q:

How would you prepare an SQL database for migration to a new server or a cloud-based service?

A:

As your company grows, you may need to expand your operations and move databases. This question gives you insight into whether the SQL applicant is familiar with migrations and how they handle them.
What to look for in an answer:

  • First-hand examples of migrating SQL databases to new locations
  • Understanding of SQL migration best practices
  • Ability to convey the SQL migration process

Example:

“I take the database through a deduplication process to remove extraneous data and ensure data quality before the migration. I also check to see if the new server meets the technical requirements for the SQL database prior to beginning the migration process.”

Q:

What are the types of normalization available in SQL and how do you use them?

A:

Normalization is an important data quality and performance tool in SQL. Applicants who know how to use this feature show that they’re experienced at maintaining SQL databases.
What to look for in an answer:

  • Definition of normalization in SQL
  • Discussion of the different SQL normalization types
  • Explanation of the most appropriate times to use each normalization type

Example:

“Normalization is a way to improve database efficiency by reducing the overall size of the database and its dependencies. I use this process to accelerate data access speed and improve overall SQL performance.”

Q:

How would you reassure a stakeholder who’s concerned about a SQL database slowdown that’s getting in the way of their work?

A:

SQL applicants may need to interact with irate users who can’t do their work or access important applications due to a database outage. The applicant needs strong customer service skills and an aptitude for explaining what’s going wrong in a way that non-technical end users can understand.
What to look for in an answer:

  • Ability to reassure end users during SQL issues
  • Knowledge of how to explain SQL problems to non-technical users
  • Skill at notifying the right people to fix SQL problems

Example:

“I understand that you’re frustrated with the system’s performance. I already reached out to the senior SQL administrator and she’s hard at work fixing the problem as we speak.”

Q:

Can you list all of the joins in SQL and the use case for each of them?

A:

This question gives the applicant a chance to demonstrate their SQL knowledge. Joins are part of basic SQL functionality, so the applicant should be able to adequately cover what they are and how they work.
What to look for in an answer:

  • A definition of a join in SQL
  • A list of all of the joins in SQL
  • How to use joins in SQL

Example:

“A join generates a set that consists of columns from tables in the database. You can use join clauses on one or more of these columns, with inner joins, full joins, left joins and right joins indicating how to combine this data.”

Q:

How would you explain the concept of a query to a non-technical audience, such as a client or end user?

A:

The SQL applicant may need to explain how they access information in a database and the basic method for doing so. When they can clearly convey their answer to a non-technical user, they have an easier time explaining what they do in presentations and other formats.
What to look for in an answer:

  • Skill at creating analogies appropriate for non-technical end users
  • Strong grasp of what a query is
  • Knowledge of query structure and usage

Example:

“A query is a way of telling the database that I want to read some of its data. I can send a request that shows me employee attendance records for the past week, for example.”

Q:

What is the importance of data integrity in your work with SQL?

A:

Monitoring, enforcing and maintaining data integrity throughout the entire data processing life cycle is one of the primary responsibilities of SQL professionals. This question helps interviewers assess a candidate's understanding of the theory behind SQL development and the practical ways they can apply that theory to their work.

A successful answer may feature:

  • The types of data integrity
  • Acknowledgement of user error
  • Key risks to data integrity and how to avoid them

Here is one example of an excellent reply to questions about the role of data integrity in successful SQL implementation:

Example:

"As an SQL professional, it is important for me to stay vigilant when confirming the accuracy of data content and location. Entity, referential, domain and user-defined data integrity are all key factors that I consider when running database tests. I try to use highly specific data constraints when coding various fields to limit the possibility of user error. I also schedule regular backups to protect from unwanted edits to the database and limit the presence of leftover test data."

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