What is SQL? Top SQL Interview Questions For Beginners

Today, almost everything in this world has tons of data about him/her, starting from a multinational organization to a new born baby.

 

Everybody relies on data and records before taking any decision related to anything anywhere. The question that arises with the given requirements for data and records is whether there must be a huge workforce required for its management as well as storage. This is where databases, database management, database management systems, and even SQL come into the frame.

 

Companies, especially big organizations, carry a huge amount of data and databases that need to be managed, maintained, manipulated, and stored; this is why database engineers are hired.

 

SQL is a language that helps us generate queries to solve real-life problems related to database management, and this is why any company wants to hire employees who have a good command of the same. And this gives birth to SQL questions being asked in company interviews.

 

However, we shall now see in detail what we mean, and what is the need to get a good hand on SQL interview questions.

 

Table of Contents

 

• What is a Database?

• What is DBMS and RDBMS?

• What is Relational Algebra?

• What is SQL?

• Why SQL in Interviews?

• Most Frequent Companies to ask for SQL Problems

• SQL Top Interview Questions

• Conclusion

 

What is a Database?

 

A database is an organized collection of structured information, or data, typically stored electronically on a computer system.

 

It is managed by a Database Management System or an RDBMS, which has various ways of being accessed, processed, or managed. Any company or organization requires these databases for their records, smooth functioning, monitoring, or, moreover, for every single activity that is carried out on a daily basis.

 

 

What are DBMS and RDBMS?

 

A Database Management System is a system software for easy, efficient, and reliable data processing and management. It can be used for:

 

• creation of a database.

• Retrieval of information from the database.

• Updating the database

• Managing a database

 

Database management is as important for an organization or any individual as the brain in the human body. Without proper management, nothing can function either on a group or an individual level, and hence, Database Management Systems are the need of the hour.

 

However, there are differences between a Database Management System (DBMS) and a Relational Database System (RDBMS) as listed below:

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RDBMS	DBMS
Data stored is in table format	Data stored is in the file format
Multiple data elements are accessible together	Individual access of data elements
Data in the form of table are linked together	No connection between data
Normalization is not achievable	There is normalization
Support distributed database	No support for distributed database
Data is stored in a large amount	Data stored is a small quantity
Oracle, SQL Server.	XML, Microsoft Access.

 

Also Read | Introduction to Database Management System

 

What is Relational Algebra?

 

Relational algebra is a procedural query language. It gives a step-by-step process to obtain the result of the query. It uses operators to perform queries. 

 

In simple terms, relational algebra is our first response to any problem related to databases. We generate the Relational Algebra solution in our heads first, and then map it with the query language. 

 

Relational Algebra is a framework for the solution to a database problem that has to be structured by any query language we are using. Relational algebra does not communicate with the RDBMS directly, but rather provides a pathway for us to frame and communicate with the RDBMS.

 

Also Read | How to Use SQL for Data Analysis?

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Approach for solving Database Problems

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What is SQL?

 

SQL is short for Structured Query Language, which is a computer language for storing, manipulating, and retrieving data stored in a relational database.

 

SQL is the most commonly used and also the standard language for relational database systems like MySQL, MS Access, Oracle, Sybase, etc.. All RDBMS rely on SQL for getting commands or inputs which is responsible for their functioning.

 

SQL is the language used to communicate with Database Management Systems, to issue commands to perform operations, to provide inputs such as data storage in databases, and much more. 

 

For instance, if in any office, the manager wants to reward all the employees with more than 90% of working days, he/she will have to look into the database of attendance and will have to communicate with the system, such as:

“Select * from employees where present>=90”.

 

Also Read | SQL: Applications, Uses, and Commands

 

Why SQL in Interviews?

 

As mentioned above, any organization, irrespective of the scale of their work, requires a proper database that is to be managed by a database management system, which is again handled by a database engineer. Hence, SQL questions and problems are asked in corporate interviews for a technical job profile and mostly for a database engineer.

 

Most companies hire database engineers for their database management, but there are companies looking for a software developer, tech analyst, or data analyst with a strong hand in SQL for multi-functioning.

 

SQL questions not only prove your skills in database handling, but they also display your analytical thinking about any problem. This is the second reason for any interviewer to ask you SQL questions in a tech interview.

 

Writing SQL queries demonstrates the reasoning skills of any person, or tells a lot about his or her IQ. It shows how a person approaches a problem and how he/she fixes it. 

 

Now let us talk about where these questions are most frequently asked or can be asked and what types of questions are asked in an interview based on SQL.

 

Also Read | SQL Tutorial for Beginners

 

Most Frequent Companies to ask for SQL Problems

 

Although SQL is a subject or a stone that can hit you anytime or anywhere, we could still speculate a little bit about where these stones are mostly thrown, or in simple terms, which companies have a tendency to dig deep into the topic, and which do not. 

 

Generally, companies having the work profile of database providers or database managers look for more and more database engineers so that the heavy computation required at their end can be taken care of by professionals. These are the companies with the top priorities of digging deep into DBMS, which automatically brings in SQL.

 

Some examples of this type of company are:

 

• Oracle RDBMS

• DBVisualizer

• SolarWinds Database Performance Analyzer

• IBM DB2

 

Second, Fintech firms have amassed a large amount of data that must be processed, manipulated, stored, or managed. These companies also require highly skilled database management at the back-end, which again calls for employees with a great hand in databases or SQL.

 

Some examples of this type of company are:

 

• Visa

• Mastercard

• Intuit

• Paypal

 

Further, e-commerce companies also have data handling capabilities at a much higher level than normal organizations. Hence, companies related to e-commerce also ask for a mastery of database knowledge and SQL problem solving.

 

Some examples of this type of company are:

 

• Amazon

• Flipkart

• Wal-Mart Stores Inc.

• Apple Inc.

 

Keeping the above information aside, even product-based companies or any company with a tech opportunity for database engineers, demand good SQL skills from their employees. If not advanced, it requires at least the basic skills that are needed to be practiced for daily actions.

 

There can be instances where the company can ask database related questions even if the job profile is not database engineering. This is done to check the technical compatibility of the candidate.

 

Some examples of this type of company are:

 

• Adobe

• Amazon

• CISCO

• Facebook (known as Meta lately)

 

We can have a fair idea about the need and frequency of encountering SQL Interview Questions. Let us now have a look at what types of questions are asked in interviews.

 

SQL Top Interview Questions 

 

• What is the difference between SQL and MySQL?

 

SQL	MySQL
SQL is a standard language which stands for Structured Query Language, based on the English language.	MySQL is a database management system.
SQL is the core of the relational database, which is used for accessing and managing databases.	MySQL is an RDMS (Relational Database Management System) such as SQL Server, Informix, etc.

 

 

• What are the different subsets of SQL?

 

1. Data Definition Language (DDL) - performing different operations on databases like; CREATE, ALTER, etc..

2. Data Manipulation Language (DML) - accessing and manipulating data such as; insert, update, etc..

3. Data Control Language (DCL) - granting control over databases like; Grant, Revoke, etc..

 

• What are Tables and Fields?

 

A table is an organized collection of data stored in the form of rows and columns. Columns can be categorized as vertical or horizontal. The columns in a table are called fields, while the rows can be referred to as records.

 

• What are Constraints in SQL?

 

Constraints are used to specify the rules concerning data in the table. It can be applied to single or multiple fields in an SQL table during the creation of the table or after it using the ALTER TABLE command. The constraints are:

 

1. NOT NULL-Restricts a NULL value from being inserted into a column.

2. CHECK-Verifies that all values in a field satisfy a condition.

3. DEFAULT-Automatically assigns a default value if no value has been specified for the field.

4. UNIQUE-Ensures unique values are inserted into the field.

5. INDEX: Indexes a field, providing faster retrieval of records.

6. APRIMARY KEY: Uniquely identifies each record in a table.

7. FOREIGN KEY-Ensures referential integrity for a record in another table.

 

• What is a Primary Key?

 

The PRIMARY KEY constraint uniquely identifies each row in a table. It must contain UNIQUE values and has an implicit NOT NULL constraint.

 

A table in SQL is strictly restricted to have one and only one primary key, which is composed of single or multiple fields (columns).

 

• What is the difference between DELETE and TRUNCATE statements?

 

The TRUNCATE command is used to delete all the rows from the table and free the space containing the table.

 

The DELETE command deletes only the rows from the table based on the condition given in the where clause or deletes all the rows from the table if no condition is specified. But it does not free the space containing the table.

 

• What is a User-defined function? What are its various types?

 

The user-defined functions in SQL are like functions in any other programming language that accept parameters, perform complex calculations, and return a value. They are written to use the logic repetitively whenever required. There are two types of SQL user-defined functions:

 

1. Scalar Function: As explained earlier, user-defined scalar functions return a single scalar value.

2. Table-Valued Functions: User-defined table-valued functions return a table as output.

 

1. Inline: returns a table data type based on a single SELECT statement.

2. Multi-statement: returns a tabular result-set but, unlike inline, multiple SELECT statements can be used inside the function body.

 

• What are the differences between OLTP and OLAP?

 

OLTP stands for Online Transaction Processing, is a class of software applications capable of supporting transaction-oriented programs. An important attribute of an OLTP system is its ability to maintain concurrency. OLTP systems often follow a decentralized architecture to avoid single points of failure. These systems are generally designed for a large audience of end-users who conduct short transactions. Queries involved in such databases are generally simple, need fast response times, and return relatively few records. A number of transactions per second acts as an effective measure for such systems.

 

OLAP stands for Online Analytical Processing, a class of software programs that are characterized by the relatively low frequency of online transactions. Queries are often too complex and involve a bunch of aggregations. For OLAP systems, the effectiveness measure relies highly on response time. Such systems are widely used for data mining or maintaining aggregated, historical data, usually in multi-dimensional schemas.

 

• What is a Stored Procedure?

 

A stored procedure is a subroutine available to applications that access a relational database management system (RDBMS). Such procedures are stored in the database data dictionary. The sole disadvantage of stored procedure is that it can be executed nowhere except in the database and occupies more memory in the database server. It also provides a sense of security and functionality as users who can't access the data directly can be granted access via stored procedures.

 

• How to create empty tables with the same structure as another table?

 

Creating empty tables with the same structure can be done smartly by fetching the records of one table into a new table using the INTO operator while fixing a WHERE clause to be false for all records. Hence, SQL prepares the new table with a duplicate structure to accept the fetched records but since no records get fetched due to the WHERE clause in action, nothing is inserted into the new table.

 

 

Conclusion

 

So far we have read about the needs of databases, database management systems, SQL, and database engineers and where they are needed. 

 

We also read some top companies asking SQL problems too often as well as specific questions (types of questions) they focus on. 

 

Now let us have a glimpse of what we read.

 

• A database is an organized collection of structured information, or data, typically stored electronically on a computer system.

• Relational algebra is a procedural query language. It gives a step-by-step process to obtain the result of the query. It uses operators to perform queries. 

• SQL is short for Structured Query Language, which is a computer language for storing, manipulating, and retrieving data stored in a relational database.

• As mentioned above, any organization, irrespective of the scale of their work, requires a proper database that is to be managed by a database management system, which is again handled by a database engineer. Hence, SQL questions and problems are asked in corporate interviews for a technical job profile and mostly for a database engineer.

 

We can see how much knowledge of databases, keeping a good hand in SQL matters in the technical world that decides our future.

 

From the above information, we can easily state that without proper data management, the technical world could crash. Hence, SQL Interview Questions become a very important part of a candidate’s portfolio and knowledge base.