Top SQL Joins Interview Questions and Answers You Should Practice

SQL Joins interview questions are among the most commonly asked in SQL Developer interviews. These questions are based on the types of SQL Joins and their applications.

An SQL Join statement joins data or rows from two or more tables based on a shared field. SQL is used in more than 1,95 lakh companies to carry out functional tasks.SQL interview questions are frequently asked in technical interviews because RDBMS is still a widely used database management system in organizations worldwide. SQL Joins questions can also be asked in data analyst, data scientist, and data engineer interviews, among other places.

Read on to learn about the topics you should be studying for your interviews, as well as the most frequently asked SQL Joins questions in FAANG interviews.

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SQL Joins interview questions account for many technical and coding SQL interviews. In this article, we will go over the most common SQL Joins interview questions that are asked of both new and experienced developers.

We will cover these topics:

• What Do SQL Developers Do?
• Topics to Prepare for Your SQL Joins Interview
• Top SQL Joins Interview Questions and Answers
• SQL Query Interview Questions on Joins and Subqueries
• SQL Joins Interview Questions for Experienced Developers
• FAQs on SQL Interviews

What Do SQL Developers do?

SQL Developers design database systems that store and access information related to a business. SQL Developers are responsible for incorporating a high level of technical know-how and data management skills to make the systems safe and secure. Their job is to prevent unauthorized access to these systems as it may put sensitive data and the business’s reputation at risk.

Besides this, SQL Developers are responsible for evaluating network infrastructure and running diagnostic tests. They need to update all information systems to ensure seamless navigation and optimal performance from time to time. To handle sensitive data related to a business, an SQL Developer must possess excellent programming skills, data engineering, and software management skills.

Topics to Prepare for Your SQL Joins Interview

Before you start with your SQL Joins interview preparation, it will be a good idea to brush up on the fundamentals. The questions asked in the tech interview are based on the following SQL Join concepts:

• Joining Tables & Queries
• Inner Join
• Left Outer Join
• Right Outer Join
• Full Outer Join
• Cross  Join
• Union
• Union All

Top SQL Join Interview Questions and Answers

SQL Joins questions asked in FAANG interviews are based on the types of SQL Joins and their applications. Besides core concepts, SQL Server interview questions are based on the position and required work experience.

Here are some SQL Join interview questions and answers commonly asked in SQL tech interviews. Make sure you practice these questions multiple times to increase your chances of cracking your interview.

Q1. What are SQL Joins?

A. An SQL Join is a clause or statement used to combine two or more tables based on common fields present in the tables.

An example of combining two tables in an organization’s database can be EmployeeInformation and EmployeeSalary. You may want to fetch the name of the highest or lowest paid employee. Now, you will find the name in the EmployeeInformation table, whereas the salary will be in the EmployeeSalary table. To obtain the necessary information, an SQL Join is used.

EmployeeInformation

EmployeeSalary

Syntax:

SELECT *

FROM EmployeeInformation INNERJOIN EmployeeSalary

ON EmployeeInformation.EmpId = EmployeeSalary.EmpId;

Q2. What are the different types of Joins in SQL?

A. The following are some types of SQL Joins:

• Left Join: It concatenates the left table rows and corresponding right table rows.
• Right Join: It concatenates the right table rows and the corresponding matching left table rows.
• Inner Join: It returns records with the same value match in both tables.
• Full Join: It returns records of both tables if there is any matching record in either table.
• Self Join: It simply joins a table to itself. Here, the table is treated like two different tables.
• Cartesian Join: It returns the rows in the first table multiplied by the rows in the second table. It is also called a cross join.

Q3. What is the difference between Full Join and Cross Join?

A Left Outer and Right Outer Join combine to make a Full Outer Join. It returns the query’s WHERE clause in all rows in both tables. When the ON condition is not satisfied, it returns a NULL value. A Cross join returns all possible combinations of all rows of both tables, resulting in a cartesian product between the two tables. The comparative difference between the two is tabulated as follows:

Q4. What is Natural Join?

A. A Natural join creates an implicit join clause between two tables based on their common attributes. A common attribute has the same name in both tables. Unlike an equi join, a Natural join does not need a comparison operator.

Let us consider an example of a Natural join. Given two tables, Employee and Department, both have a common attribute, ‘EmpNo.’

Employees

Department

Syntax:

SELECT Employees.Name, Department.DName

FROM Department

Natural JOIN Employees

Q5. What is an Equi Join?

Ans. It is a type of join that combines tables based on matching values in the specified columns. Some features of equi join are:

• Column names need not be the same.
• The resulting table sometimes contains repeated columns.
• An equi join can be performed on more than two tables.

Syntax:

SELECT *

FROM TableNameA

JOIN TableNameB

ON TableNameA.ColumnName = TableNameB.ColumnName;

Here are some Amazon SQL Interview Questions for more practice.

SQL Query Interview Questions on Joins

These are some of the most frequently asked SQL Query interview questions on Joins:

1. What is the significance of SQL Joins in database management?
2. What do you mean by Nested Joins in SQL?
3. What do you understand about Merge Join in SQL?
4. What is a Hash Join?
5. How will you structure data to perform a Join Operation in a one-to-many relationship situation?
6. Differentiate between Inner Join and Left Join.
7. Differentiate between Left Join and Right Join.
8. What do you understand about a Non-equi Join?
9. Differentiate between Cross Join and Natural Join.
10. How is a Union clause different from a Join?
11. Write an SQL syntax for joining 3 tables.
12. Differentiate between an SQL Left Join and an SQL Inner Join.
13. How will you join more than two tables?
14. How will you join a table to itself?

SQL Interview Questions on Joins and Subqueries

Here are some SQL Join interview questions on Joins and Subqueries:

1. What is Subquery?
2. What are the advantages and disadvantages of Subquery in SQL?
3. List down the important rules for Subqueries.
4. Differentiate between Nested Subquery, Correlated Subquery, and Join Operation.
5. Give an application of Join Operation and Subquery.
6. How will you use the IN Operator with a SubQuery in SQL?
7. Describe subqueries with the INSERT statement.
8. Explain subqueries with the UPDATE statement.

Also, check out the most frequently asked Facebook SQL Interview Questions here.

SQL Joins Interview Questions for Experienced Developers

Take a look at these SQL Joins interview questions for experienced professionals:

1. Is it necessary that the JOIN condition should be based on equality?
2. How will you structure data for facilitating Join in a one-to-many relationship condition?
3. How will you structure data for facilitating Join in a many-to-many relationship condition?
4. What is going to be the outcome of this Inner Join Query:

SELECT a.Col1, b.Col2

   FROM tbl_samplejoin1 a INNER JOIN tbl_samplejoin2 b

   ON a.Col1= b.Col2

Recommended Reading: 50+ SQL Interview Questions and Tips on How to Crack Them

FAQs on SQL Joins Interview Questions

Q1. What are the most important SQL Join topics to prepare for SQL interview?

‍Questions asked in SQL interviews are based on SQL Join topics - Joining Tables & Queries, Inner Join, Left Outer Join, Right Outer Join, Full Outer Join, Cross  Join, Union, and Union All. There is a special focus on the application of SQL Join in various practical scenarios.

Q2. Which companies use SQL?

‍A. Some of the major companies that use SQL are Stack Overflow, Accenture, Microsoft, Cognizant, Dell, Alibaba Travels, Paralect, and many more.

Q3. What is CTE SQL?

CTE stands for Common Table Expression. It is a temporary named result set used to reference a SELECT, UPDATE, INSERT, or DELETE statement.

Q4. How do you explain SQL joins in an interview?

‍The simplest way to explain SQL Joins in an interview is by talking about its basic function of combining data from two or more tables in SQL. For this, all the tables involved should have at least one common data column.

Q5. What are the top SQL Joins interview questions?

The difficulty of SQL Joins interview questions differs based on your experience level. The most commonly asked SQL Joins interview questions are: What are SQL Joins? What are the different types of Joins in SQL? What is the difference between Full Join and Cross Join? What is the difference between Full Join and Cross Join?

Q6. What is the salary of an SQL Developer?‍

The average salary of an SQL Developer is approximately $87,398 per year.

Crack Your Next SQL Joins Interview

Nail your SQL Joins interview by preparing every aspect from specific SQL Joins interview questions and SQL Joins applications, along with in-depth preparation for behavioral and leadership interview rounds.

Experts recommend practising mock interviews and SQL interview questions before you sit for the actual interview. It will boost your confidence and prepare you for all kinds of possible scenarios you can face in the actual interview.

If you prepare the right way, guided by FAANG interview experts, you can easily crack your next SQL Joins interview. Register for IK’s webinar today to leverage exclusive interview preparation courses and mock interviews with experts.

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