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A Comprehensive List of SQL Interview Questions

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SQL is a powerful query language that is used to manipulate and query data in relational databases. It is one of the most popular database languages in use today, and it is essential for anyone who wants to work in the field of data analytics or database administration.
If you are looking for a job in the field of data analytics or database administration, then you will likely need to pass an SQL interview. The questions can be difficult, and it is important to be prepared. In this article, we will provide a comprehensive list of SQL interview questions and answers. We will also provide tips for how to best answer these questions.

What Is SQL and How Does It Work?

SQL is a database query language.
It allows you to easily manipulate and query data from a database. SQL works by reading a database schema and creating a query plan, which is then executed to return the desired results.
SQL is used in a wide range of applications, from small websites to large corporate databases. It is a very versatile tool and is essential for any database administrator or developer.

What Is the Purpose of Using DDL and DML Commands in SQL?

The two main types of SQL commands are DDL and DML.
DDL, or Data Definition Language, is used to define the structure of a database. This includes the creation of tables, columns, and constraints.
DML, or Data Manipulation Language, is used to insert, update, and delete data from a database.

How Do You Find Duplicate Records in a Table?

You can find duplicate records in a table by using the following SQL statement:
SELECT * FROM table_name WHERE column_1 = column_2;

What Are the Different Types of Joins in SQL?

Knowing the different types of Joins in SQL is essential for any SQL interview. There are four main types of Joins: inner, left, right, and full outer join. An inner join is used to match rows from two tables that have the same value in a given column. A left join will include all records from the left table but only matching records from the right table and vice versa for a right join. Lastly, a full outer join will include all records from both tables. Knowing these different types of Joins will prove your knowledge of SQL and put you ahead of other candidates.

What Is an Index and Why Should You Use Them?

When you are asked about indexes in an interview, it is important to remember that an index is a data structure used to improve the speed of data retrieval operations on a database table by reducing the number of disk accesses required when a query is processed. You should use indexes whenever possible, because they can significantly speed up search, join, and order by operations. Furthermore, it is much faster to scan the index rather than scanning through all of the records in the table. Indexes also help with query optimization since they help reduce the cost of executing queries.

What Are the Most Common SQL Performance Optimization Techniques?

When it comes to performance optimization, there are several techniques that you should be aware of. Indexing is one of the most common SQL optimization techniques. Indexing allows you to quickly search for records that match a given criteria, which can lead to faster query processing. Another popular technique is normalization, which helps with data maintenance by breaking down larger data sets into smaller, logical pieces. 

Lastly, stored procedures can also be used to increase performance as they allow you to store logic in the database instead of in application code. Being able to answer questions like these will give you a leg up in the interview and show that you understand the concepts and techniques behind SQL performance optimization.


But don’t just stop at practicing your answers to these questions. In addition to thoroughly understanding the concepts behind the questions, you should also be able to speak to your experience with SQL in a way that is clear, concise, and convincing. And, of course, don’t forget the basics like making sure you arrive on time, dress the part, and exude confidence. With a little preparation, you’ll be nailing SQL interviews in no time.