SQL Interview Questions

SQL is considered as one of the most powerful tools to handle and manage and also to manipulate databases, it enables the users to interact with databases to retrieve, insert and delete data. SQL is an essential skill for any individual who is working with the data, as it is used and adapted in different roles including database administrators, analysts of data and also the software developers.

Whether you are a beginner or an expert professional, it is essential to be well prepared for SQL questions based interview questions, then this blog post will help you to cover various SQL interview questions with the answers to help you to ace your next interview.

Introduction

The interview questions of the SQL will assess your ability to work with the databases which are relational. They will cover all the aspects related to the basic questions (SELECT, WHERE and JOIN), manipulation of the data (INSERT, UPDATE and DELETE) and database design (Primary keys). 

Advanced questions may include subqueries, views, procedures which are stored and techniques of performance optimization including indexing. These questions evaluate both your theoretical knowledge and practical skills while solving the real world problems related to the databases

Top 12 Trending SQL Interview Questions

• What are different types of Joins and examples of it?

• Explain the primary key in SQL?

• Explain the trigger in SQL?

• Explain what is a transaction in SQL?

• Explain the view option in SQL?

• Explain how to prevent SQL injection?

• Explain the Indexes in SQL?

• Explain the major difference between ‘WHERE’ & ‘HAVING’ in SQL?

• Explain the normalization and denormalization in SQL?

• Explain the ‘Subquery’ with a suitable example?

• Explain in detail the importance of ‘INDEXES’ in SQL?

• Explain the aggregate functions in SQL?

Beginner Level Interview Questions

• What is SQL? 

SQL is long termed as a Structured Query Language and it is used and adapted to communicate with the relational databases. It consists of the standardized way to interact with the databases, enabling users to perform different operations on the data, consisting of retrieval, insertion, updating and also for deletion.

• Explain the various types of Commands in SQL?

SELECT: It retrieves data from the database

INSERT: It adds new records to the table

UPDATE: It modifies the existing records in a table

DELETE: It removes record from the table

CREATE: It modifies the existing database structure of an object

ALTER: It modifies the existing structure of the database

DROP: It deletes an existing database object

• Explain the difference between ‘WHERE’ & ‘HAVING’?

Before grouping, the rows are filtered using ‘WHERE’ (it acts only on individual records). However, groupings are filtered using ‘HAVING’.

Intermediate Level Interview Questions

• What is Normalization in SQL? Explain why it is important?

The process of normalization consists of breaking up large tables into smaller ones in order to minimize the redundancy and dependence. This makes updates to be more effective, enhances data integrity & minimizes data inconsistencies.

• Explain the aggregate in SQL?

Aggregate functions produce a single result after performing computations on a collection of values. Among the most frequently used aggregate functions are:

COUNT ( ): It counts the total number of rows

SUM ( ): It adds up values

AVG ( ): It computes the average values

MIN ( ): It returns the lowest value

MAX ( ): It returns the highest value

• What is Subquery, explain it with an example?

A query inside another query is known as a subquery. It is frequently used to compute values or during filtering of the data

For example:

SELECT * FROM employees WHERE salary > (SELECT  AVG (salary)  FROM employees))

Advance Level Interview Questions

• Explain the Trigger in SQL?

Whenever specific events including the (INSERT, UPDATE, DELETE) occur on a table, a trigger is a collection of SQL statements that start running automatically. Triggers are employed in data synchronization, business rule implementation as well as in  auditing.

• What is a deadlock in SQL? How to resolve it?

When two or more transactions are waiting on one another to release the resources, the database freezes, which is known as a deadlock. 

The Deadlock can be resolved by following:

Transaction Management: Ensuring that the transactions should be as short as possible

Locking the Mechanism: Implementing the correct and appropriate locking mechanism

Deadlock Detection: Many Databases automatically find the deadlocks and eliminate the one of the transactions to solve the error occurred

• How to optimize the slow SQL query?

Actually there are many ways to optimize the slow SQL query, including:

Indexing: Use the INDEXES for speeding up lookups on most queried column

Refactoring of Query: Avoid unnecessary joins, reduce the number of subqueries, and rewrite complicated queries

EXPLAIN plan: To examine the query execution plan and locate bottlenecks, use the command called ‘EXPLAIN’

Conclusion

Especially success in data-driven fields requires a powerful understanding of SQL, and being ready for SQL interview questions will make you stand out during the process. You will stand out if you have an excellent understanding of SQL concepts and ideas, from simple questions and doubts to complicated performance improvement strategies, regardless of your level of expertise.

Look into taking a SQL course to leverage and increase your understanding of the SQL and improve your performance during the time of  interviews. An SQL training course can improve your comprehension and problem-solving skills by offering practical experience and professional insights and an amount of level of experience

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