14 Popular SQL Interview Questions for Freshers

Structured Query Language (SQL) is a domain-specific language used in programming and designed for managing data held in a Relational Database Management System (RDBMS).

SQL is an essential skill for job seekers in the tech industry. It is a highly sought-after skill in the job market, and proficiency in SQL can open up a variety of job opportunities for job seekers.

In today's digital age, data has become a valuable commodity, and companies are always looking for ways to collect, analyze, and store data efficiently.

SQL is one of the most widely used languages for managing data, making it a critical skill for job seekers looking to work in the tech industry.

Whether you are a data analyst, software developer, or database administrator, having a good understanding of SQL is essential to succeed in your career.

In the next sections, we will explore some of the most important concepts in SQL that job seekers should know to succeed in the tech industry. These concepts include primary keys, joins, subqueries, views, normalization, and stored procedures.

By the end of this blog, you will have a good understanding of these concepts and how they can be applied in the real world.

1. What are SQL and its basic syntax?
2. What is a database and what are its types?
3. What are the primary key and foreign key?
4. What are normalization and its types?
5. What are a join and its types?
6. What are some common SQL functions?
7. What are some common SQL functions?
8. What is SQL and why is it important in the IT industry?
9. What is SQL and what is its basic syntax?
10. What is a database and what are its types?
11. What are the primary key and foreign key?
12. What are normalization and its types?
13. What are a join and its types?
14. What are a subquery and its types?

What are SQL and its basic syntax?

SQL or Structured Query Language is a programming language used to manage and manipulate relational databases.

It is a standard language for managing data stored in relational database management systems (RDBMS) such as MySQL, Oracle, and Microsoft SQL Server.

SQL consists of several components, including data definition language (DDL), data manipulation language (DML), data control language (DCL), and transaction control language (TCL).

DDL is used to create and modify database objects such as tables, indexes, and views. DML is used to insert, update, and delete data from tables. DCL is used to control access to data in the database, while TCL is used to manage transactions.

Here's an example of basic SQL syntax for creating a table:

CREATE TABLE employees ( 
id INT PRIMARY KEY, 
name VARCHAR(50), 
department VARCHAR(50), 
salary INT 
); 

In this example, we're creating a table named "employees" with four columns: id, name, department, and salary. The "id" column is defined as the primary key, indicating that it uniquely identifies each row in the table.

SQL is used in databases to store and retrieve data. With SQL, you can perform various operations on data, such as selecting, inserting, updating, and deleting. SQL also allows you to join multiple tables, filter data based on certain conditions, and aggregate data using functions.

In conclusion, having a good understanding of SQL and its basic syntax is crucial for anyone looking to work with databases.

It is important to be familiar with the different components of SQL and how they are used in database management. In the next section, we'll explore what a database is and its types.

What is a database and what are its types?

A database is a structured collection of data, stored and organized to allow easy access and retrieval. There are two main types of databases: relational and non-relational.

Relational databases are the most commonly used type of database. They are based on the relational model and store data in tables that are related to each other through primary and foreign keys. Examples of relational databases include MySQL, Oracle, and SQL Server.

On the other hand, non-relational databases, also known as NoSQL databases, do not use the traditional table-based structure. Instead, they store data in a variety of ways, such as key-value pairs, documents, or graphs.

Examples of non-relational databases include MongoDB, Cassandra, and Redis.

The main difference between relational and non-relational databases is the way they store and retrieve data. Relational databases are better suited for structured data that requires complex queries, while non-relational databases are better suited for unstructured data that require high scalability and flexibility.

In conclusion, understanding the different types of databases is important for SQL interviews as it shows your knowledge of the basics of database management.

It also helps you choose the right database for your project based on its requirements and constraints.

What are the primary key and foreign key?

A primary key is a unique identifier for a record in a database table. It is a column or a set of columns that uniquely identifies each row in the table. A primary key cannot be null and must be unique.

Primary keys are used to enforce data integrity and ensure that each record in a table is unique.

A foreign key is a column or a set of columns in a table that refers to the primary key of another table. It is used to establish a relationship between two tables.

The foreign key in one table refers to the primary key in another table. This relationship is used to ensure data integrity and maintain consistency in the database.

For example, consider two tables, one for customers and another for orders. The customer table has a primary key column called customer_id. The order table has a foreign key column called customer_id, which refers to the customer_id column in the customer table. This relationship ensures that each order is associated with a valid customer in the customer table.

In summary, primary keys and foreign keys are important concepts in database design and are used to ensure data integrity and maintain consistency in the database.

Job seekers need to understand these concepts and be able to apply them in SQL queries and database design.

What are normalization and its types?

Normalization is the process of organizing data in a database to minimize redundancy and dependency. It is an essential part of database design as it ensures data consistency and accuracy.

There are three types of normalization - First Normal Form (1NF), Second Normal Form (2NF), and Third Normal Form (3NF).

First Normal Form (1NF): This type of normalization ensures that each column in a table contains atomic values, meaning that each cell must contain only one value. It eliminates repeating groups and ensures that each row is unique.

Second Normal Form (2NF): This type of normalization eliminates partial dependencies, meaning that each non-key column in a table is dependent on the primary key. It ensures that each column in a table is related to the primary key.

Third Normal Form (3NF): This type of normalization eliminates transitive dependencies, meaning that each non-key column in a table is not dependent on another non-key column. It ensures that each column in a table is directly related to the primary key.

For example, consider a table that contains customer details and their orders. The table may contain columns such as customer ID, customer name, order ID, order date, order amount, and product ID.

To normalize this table, we can create two tables - one for customer details and another for orders. The customer details table will contain columns such as customer ID and customer name, while the orders table will contain columns such as order ID, order date, order amount, product ID, and customer ID (as a foreign key).

Normalization helps in reducing data redundancy, improving data consistency, and making it easier to maintain and update the database.

What are a join and its types?

Join is one of the most important concepts in SQL, used to combine data from two or more tables. It is a powerful tool that allows the user to extract meaningful information from a database.

There are three types of joins: Inner join, Left join, and Right join.

Inner join returns only the matching rows from both tables. It is used when we want to extract data from two tables that have common fields.

For example, if we have a table of employees and a table of departments, we can use an inner join to get the names of employees and their respective departments.

Left join returns all the rows from the left table and the matching rows from the right table. If there is no match in the right table, the result will be NULL. It is used when we want to extract all the data from the left table and only the matching data from the right table.

For example, if we have a table of employees and a table of departments, we can use left join to get the names of all employees along with their respective departments (even if some employees do not belong to any department).

Right, join returns all the rows from the right table and the matching rows from the left table. If there is no match in the left table, the result will be NULL. It is used when we want to extract all the data from the right table and only the matching data from the left table.

For example, if we have a table of departments and a table of employees, we can use right join to get the names of all departments along with their respective employees (even if some departments do not have any employees).

To illustrate the concept of joining, let's consider the following example:

Table 1: Employees

Table 2: Departments

Inner join query:

SELECT Employees.EmpName, Departments.DeptName
FROM Employees
INNER JOIN Departments
ON Employees.DeptID = Departments.DeptID;

Result:

Left join query:

SELECT Employees.EmpName, Departments.DeptName
FROM Employees
LEFT JOIN Departments
ON Employees.DeptID = Departments.DeptID;

Result:

Right join query:

SELECT Employees.EmpName, Departments.DeptName
FROM Employees
RIGHT JOIN Departments
ON Employees.DeptID = Departments.DeptID;

Result:

What are some common SQL functions?

SQL functions are used to perform calculations on data stored in a database. They are an essential part of SQL queries and help in retrieving specific information from a database. Here are some common SQL functions that every fresher should know:

1. COUNT: This function is used to count the number of rows in a table. For example, if we want to count the number of employees in a company, we can use the following query:

SELECT COUNT(*) FROM employees;

1. AVG: This function is used to calculate the average value of a column in a table. For example, if we want to calculate the average salary of employees in a company, we can use the following query:

SELECT AVG(salary) FROM employees;

1. MAX: This function is used to find the maximum value in a column in a table. For example, if we want to find the highest salary of an employee in a company, we can use the following query:

SELECT MAX(salary) FROM employees;

1. MIN: This function is used to find the minimum value in a column in a table. For example, if we want to find the lowest salary of an employee in a company, we can use the following query:

SELECT MIN(salary) FROM employees;

These functions can be used in combination with other SQL statements to get the desired result. For example, if we want to find the number of employees who earn more than the average salary, we can use the following query:

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

Knowing these common SQL functions will help freshers in preparing for SQL interviews and in writing efficient SQL queries.

What are some common SQL functions?

SQL functions are built-in commands that are used to perform specific operations on data in a database. These functions make it easier to manipulate data and retrieve the desired results.

Here are some of the most common SQL functions:

1. COUNT - This function is used to count the number of rows in a table. For example, to count the number of employees in a company, you can use the following query:

SELECT COUNT(*) FROM employees;

1. AVG - This function is used to calculate the average value of a column. For example, to calculate the average salary of employees, you can use the following query:

SELECT AVG(salary) FROM employees;

1. MAX - This function is used to find the maximum value in a column. For example, to find the highest salary in a company, you can use the following query:

SELECT MAX(salary) FROM employees;

1. MIN - This function is used to find the minimum value in a column. For example, to find the lowest salary in a company, you can use the following query:

SELECT MIN(salary) FROM employees;

These functions can be used in combination with other SQL commands to perform complex operations on data. For instance, you can use the COUNT function with the WHERE clause to count the number of employees in a specific department.

Similarly, you can use the AVG function with the GROUP BY clause to calculate the average salary of employees in each department.

In conclusion, SQL functions are an essential part of SQL queries. By understanding these functions and how to use them, you can write efficient and effective SQL queries that retrieve the desired results from a database.

What is SQL and why is it important in the IT industry?

SQL (Structured Query Language) is a programming language used to manage and manipulate data in relational databases.

It is important in the IT industry because it allows developers to create, modify, and manage databases, as well as retrieve and manipulate data from those databases.

SQL is used in a wide range of applications, including e-commerce, finance, healthcare, and more.

What is SQL and what is its basic syntax?

SQL is a programming language used to manage and manipulate data in relational databases. Its main components include SELECT, FROM, WHERE, and ORDER BY.

For example, a basic SQL statement to retrieve all records from a table named "employees" would look like this:

SELECT * FROM employees;

This statement selects all columns and rows from the "employees" table.

What is a database and what are its types?

A database is a collection of data that is organized and stored in a structured manner for easy retrieval and management.

There are two main types of databases: Relational and Non-Relational.

Relational databases store data in tables with relationships between them, while non-relational databases store data in other formats, such as documents or graphs.

What are the primary key and foreign key?

A primary key is a unique identifier for a record in a database table. It is used to ensure that each record in the table is unique and can be accessed quickly.

A foreign key is a field in one table that refers to the primary key in another table. It is used to create relationships between tables and ensure data integrity.

For example, if we have two tables, "employees" and "departments," the "department_id" field in the "employees" table could be a foreign key that refers to the "id" field in the "departments" table.

What are normalization and its types?

Normalization is the process of organizing data in a database to minimize redundancy and improve data integrity. There are three types of normalization: first normal form (1NF), second normal form (2NF), and third normal form (3NF).

1NF involves removing repeating groups and ensuring that each column contains atomic values. 2NF involves removing partial dependencies, where a non-key column depends on only part of the primary key. 3NF involves removing transitive dependencies, where a non-key column depends on another non-key column.

For example, if we have a table named "orders" with columns for "order_id," "customer_id," "customer_name," and "customer_address," we could normalize it by creating a separate table for customers and linking it to the orders table via the "customer_id" field.

What are a join and its types?

A join is a way to combine data from two or more tables based on a common column or relationship. There are three types of join: inner join, left join, and right join.

An inner join returns only the rows that have matching values in both tables. A left join returns all the rows from the left table and matching rows from the right table. A right join returns all the rows from the right table and matching rows from the left table.

For example, if we have two tables named "employees" and "departments," we could join them on the "department_id" field to get a list of all employees and their corresponding departments.

What are a subquery and its types?

A subquery is a query within another query. There are two types of subqueries: correlated and non-correlated.

A non-correlated subquery is a subquery that can be run independently of the main query and returns a single value. A correlated subquery is a subquery that depends on the main query and returns a value for each row in the main query.

For example, if we have a table named "orders" and want to find the average order value for each customer, we could use a correlated subquery to calculate the average for each customer.

Conclusion

Preparing for a job interview is tough, especially when technicalities are involved. We hope these basic SQL interview questions were helpful.

Remember to polish up your knowledge of the technical aspects to ace your SQL interview questions for freshers.

All the Best!