CodeSubmit Interview Series

The 7 Best SQL Interview Questions in 2023

What are the top SQL interview questions? Here are the best questions to ask developers at your next interview. 

Want to learn more? Read on! 

Interviewing SQL developers 

SQL, or Structured Query Language, is a domain-specific language that’s used to manage data in a relational database management system or for stream processing in a relational data stream management system. 

SQL developers design, create, and maintain databases. For instance, they build and validate databases’ security and efficiency, create program views, functions, and stored procedures, and write optimized SQL queries for integrations with other applications.

What you should look for in a SQL developer is experience working in a development environment like Oracle SQL Developer and knowledge of relevant languages such as Python, C#, or Java. If they work with MySQL, they need experience with PHP. Also, a background in Unix is a major plus. 

But how do you find the best SQL developers? Here’s what you need to know. 

Beginner SQL interview questions

What are the top junior SQL interview questions? Here you go! 

1. What is a database? 


A database is an organized collection of structured data. This data can be stored, accessed, managed, and retrieved from a remote or local computer system. Bigger databases are hosted on computer clusters and cloud storage, while smaller databases can be stored on a file system.

2. Explain the different subsets of SQL. 


The different subsets of SQL are: 

  • Data Definition Language (DDL) - DDL allows you to perform various operations on the database (CREATE, ALTER, DELETE objects). 

  • Data Control Language (DCL) - DCL allows you to control access to the database. 

  • Data Manipulation Language (DML) - you can access and manipulate data (insert, update, delete, and retrieve data from the database)

3. What are DBMS and RDBMS? 


A Database Management System (DBMS) is a software application that interacts with the database, applications, and users to capture and analyze data. It allows users to modify, retrieve, and delete data in databases. 

There are two types of DBMS. The first is the Relational Database Management System - one example is MySQL. The data is stored in relations (tables). The other type is the Non-Relational Database Management System. One example is MongoDB; there is no concept of relations, attributes, and tuples. 

A Relational Database Management System (RDBMS), on the other hand, is a set of applications and features. These allow IT professionals and developers to edit, administer and interact with relational databases, often with the help of Structured Query Language (SQL). RDBMS is the most used database system in the world. 

4. Explain normalization and describe the different types. 


Normalization is used to reduce data redundancy and dependency. This is done by organizing fields and tables in databases and it involves constructing tables and setting up relationships between tables according to specific rules, which remove redundancy and inconsistent dependency. 

The different types of normalization are: 

  • First Normal Form: The relation is in the first normal form if every attribute is single-valued. But it is in violation of the first normal form if it contains a composite or multi-varied attribute. 

  • Second Normal Form: A relation is in second normal form if every attribute is single-valued and it doesn’t have any partial dependency. In other words, it doesn’t have a non-prime attribute that relies on any proper subset of a candidate key of the table. 

  • Third Normal Form: If a relation meets the conditions of the second normal form and there isn’t any transitive dependency between the non-prime attributes, it’s in the third normal form. 

  • Boyce-Codd Normal Form: If a relation meets the conditions of the third normal form and the left-hand side is a super key for every functional dependency.

Advanced SQL interview questions

What are the top senior SQL interview questions? Take a look! 

5. Define an execution plan and explain when you would use it. 


An execution plan is a road map that graphically or textually shows the data retrieval methods that the SQL server’s query optimizer has chosen for a stored procedure or ad hoc query. 

Execution plans can help in understanding and analyzing the performance characteristics of a query or stored procedure because the plan is used to execute the query or stored procedure. 

SQL systems tend to help you obtain textual execution plans by using a keyword like EXPLAIN. Microsoft SQL Servers often have an option called “Show Execution Plan” and if this is turned on, the query execution plan will show in a separate window. 

6. Explain the difference between Cluster and Non-Cluster Index. 


A Clustered Index is used for easy retrieval of data from the database by altering how records are stored. The database sorts rows by the column which is set to be clustered indexed. 

A Non-Clustered Index creates a separate object within a table instead of altering the way it was stored. It points back to the original table rows after searching. 

7. Define ACID in a database. 


ACID refers to atomicity, consistency, isolation, and durability; these are properties that are used to check the reliability of transactions. 

Atomicity refers to completed or failed transactions (transaction refers to a single logical operation on data). If an aspect of a transaction fails, the whole transaction fails and the state of the database is unchanged. 

Consistency refers to when the data meets all validity guidelines. 

The objective of isolation is concurrency management. 

And durability means that once a transaction is committed, it will occur despite what happens in terms of a power outage or other disturbances.  

Over to you!

There you have it! Now you know what the top SQL interview questions are.

However, your interviews are just one part of the process. You also need to assess your developers.

The best way is to use assessment and live coding tests. 

And that’s something CodeSubmit helps you with. Our assessment challenges and live coding tests are designed for your SQL interviews.

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