Top 20 SQL JOINs Interview Questions and Answers (2023)

Data is very valuable to organizations. Actionable insights that give an organization competitive advantage and help it run more efficiently can be extracted from the organization's data. Therefore, data must be collected and stored.

Databases are an organized way to store and query data. There are two main types of databases: relational and non-relational.

Relational databases are very popular because they're structured and data is organized in tables. The data in the various tables may have one-to-one, one-to-many, or many-to-many relationships. The way the data is organized in a relational database is defined in its schema.

Non-relational databases store data using simple key-value pairs in non-tabular form. Although relational databases support Binary Large Objects (BLOB) for storing semi-structured and unstructed data, storing and retrieving these types of data is easier with non-relational databases.

The Structured Query Language (SQL) is the language for accessing and interacting with relational databases. If you want to add, delete, edit, or query information on a relational database, the easiest way to do it is through SQL.

Relational databases dominate the database market and they are projected to grow by more than 30 percent between 2021 and 2026. SQL is the most in-demand skill for data related jobs. As more and more organizations embrace the use of relational databases, the demand for data professionals with SQL skills will continue to grow.

More often than not, you may need to combine two or more tables in a relational database to get the data needed for performing your analysis. You can combine tables in SQL with JOIN clauses. There are several SQL JOIN clauses; understanding how each of them works can be challenging.

SQL JOIN questions are, therefore, interviewers' favorites. They tend to show up in most SQL interviews. In this tutorial, we'll take you step-by-step through the answers to the top 20 SQL JOIN interview questions — and equip you with the knowledge to ace your upcoming SQL interviews. We'll be writing a lot of SQL queries. This SQL Cheat Sheet will help you get up to speed if your SQL skills are a little rusty.

SQL JOINs Interview Questions

In a SQL interview, you'll most likely be asked questions that require you to combine tables. SQL JOIN clauses are used to combine data from two or more tables in a relational database.

SQL JOIN clauses are often used when the tables share some sort of relationship. The most common conditional expression to join tables is the equality conditional (equi-join). However, you can join tables that don't share any relationships and use other conditional expressions besides equality.

Here are some commoly asked SQL JOIN interview questions and their answers. We recommend that you draw your own diagrams and repeatedly practice these questions to master how SQL JOINs work.

1. What are SQL JOINs?

Answer

SQL JOIN clauses are used to combine rows from two or more tables. Usually, tables are joined based on a related column between them, but you can also join tables that have no relationships. The term "SQL JOIN" is used to refer to a SQL query with the JOIN keyword. This is known as an explicit join. However, you can also join tables in SQL without using the JOIN keyword. This is known as an implicit join.

2. What are the general explicit and implicit join notation syntaxes?

Answer

EXPLICIT JOIN:

SELECT *
FROM [left table]
[JOIN CLAUSE] [right table]
ON conditional expression;

IMPLICIT JOIN:

SELECT *
FROM [left table], [right table]
WHERE conditional expression;

The conditional expression is not required for some types of SQL JOINs.

3. What are the different types of JOINs in SQL?

Answer

The main types of SQL JOINs are:

• CROSS JOIN: matches every row of the left table with every row of the right table and returns their Cartesian product. It's also known as Cartesian JOIN.
• [INNER] JOIN: returns rows that have matching values on both the right and left tables, based on the specified conditional expression.
• NATURAL JOIN: a type of equi-join that combines the left and right tables using common column names.
• LEFT [OUTER] JOIN: returns all the rows from the left table with the matching rows from the right table, based on the specified conditional expression.
• RIGHT [OUTER] JOIN: returns all the rows from the right table with the matching rows from the left table, based on the specified conditional expression.
• FULL [OUTER] JOIN: returns all rows from the left and the right tables, regardless of whether the conditional expression is matched.
• SELF JOIN: allows you to join a table to itself as if the tables were two different tables.

It's worth noting that the SQL syntax placed inside square brackets is optional and can be excluded from the query. For example, the SQL query below executes an INNER JOIN:

SELECT *
FROM left_table
JOIN right_table
ON conditional expression;

4. What type of JOIN is executed without conditional expression in the explict and implict join notations shown below?

Explicit join:

SELECT *
FROM [left table]
[JOIN CLAUSE] [right table]

Implict join:

SELECT *
FROM [left table], [right table]

Answer

Let's set up a SQLite database with tables. When you try to connect to a database that doesn't exist, it's created for you. First:

pip install ipython-sql

Then create the database and tables:

     * sqlite:///joins.db
    Done.
    1 rows affected.
    1 rows affected.
    1 rows affected.
    1 rows affected.
    Done.
    1 rows affected.
    1 rows affected.
    1 rows affected.
    Done.
    1 rows affected.
    1 rows affected.
    1 rows affected.
    Done.
    1 rows affected.
    1 rows affected.
    1 rows affected.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.
    1 rows affected.
    1 rows affected.
    1 rows affected.
    1 rows affected.
    1 rows affected.
    Done.
    1 rows affected.
    1 rows affected.
    1 rows affected.
    1 rows affected.
    1 rows affected.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

SELECT *
FROM left_table
[JOIN CLAUSE] right_table
    USING [col_name, ..];

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.
    Done.

     * sqlite:///joins.db
    Done.
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

     * sqlite:///joins.db
    Done.

16. How will you structure data for facilitating JOIN in a one-to-many relationship condition?
Answer
In a one-to-many relationship, one record in table A can be associated to more than one record in table B. So the primary key of A is a foreign key in B. The students and last_enrolment tables share a one-to-many relationship, as a student can be enrolled in more than one course.
-- create a students table:

CREATE TABLE students (
    student_id INTEGER PRIMARY KEY,
    first_name TEXT NOT NULL,
    last_name TEXT NOT NULL
)

--create last_enrolment table

CREATE TABLE last_enrolment (
    student_id,
    course_id,
    FOREIGN KEY (student_id) REFERENCES students(student_id),
    FOREIGN KEY (course_id) REFERENCES courses(course_id)
);
In the above code snippet, the student_id PRIMARY KEY in the students table is set as FOREIGN KEY in the last_enrolment table.
17. How will you structure data for facilitating JOIN in a many-to-many relationship condition?
Answer
In a many-to-many relationship, multiple records in table A are associated with multiple records in table B. There is a many-to-many relationship between the students and courses tables. A student can take more than one course, and there can be more than one student in a course.
The many-to-many relationship in our example is broken into two one-to-many relationships. The first one-to-many relationship is the relationship between students and last_enrolment, and the second one-to-many relationship is the relationship between courses and last_enrolment. The last_enrolment table connects the students and courses tables and is referred to as a join table.
-- create a students table:

CREATE TABLE students (
    student_id INTEGER PRIMARY KEY,
    first_name TEXT NOT NULL,
    last_name TEXT NOT NULL
)

-- CREATE the courses table

CREATE TABLE courses (
    course_id INTEGER PRIMARY KEY,
    course_name TEXT NOT NULL
)

--create last_enrolment table

CREATE TABLE last_enrolment (
    student_id,
    course_id,
    FOREIGN KEY (student_id) REFERENCES students(student_id),
    FOREIGN KEY (course_id) REFERENCES courses(course_id)
);
18. Write a query to return information on people that love Jazz music
Write a query to return the email and full_names of people that have bought Jazz music. You'll be working with the chinook.db. You can find the schema and download the file from Answer

First, we connect to the database and list the tables in it:

     * sqlite:///chinook.db
    Done.


name


album


artist


customer


employee


genre


invoice


invoice_line


media_type


playlist


playlist_track


track


We'll return the email, first_name, and last_name columns on the customer table for the subset where the genre is Jazz. We want to connect the customer table to the genre table. The schema will be helpful here. In the schema, we'll need to join the customer, invoice, invoice_line, track, and genre tables to get the information we want.

     * sqlite:///chinook.db
    Done.


email
full_name


[email protected]
Aaron Mitchell


[email protected]
Astrid Gruber


[email protected]
Bjørn Hansen


[email protected]
Camille Bernard


[email protected]
Daan Peeters


[email protected]
Dan Miller


[email protected]
Edward Francis


[email protected]
Eduardo Martins


[email protected]
Ellie Sullivan


[email protected]
Enrique Muñoz


[email protected]
Fernanda Ramos


[email protected]
František Wichterlová


[email protected]
François Tremblay


[email protected]
Hannah Schneider


[email protected]
Helena Holý


[email protected]
Heather Leacock


[email protected]
Isabelle Mercier


[email protected]
Jack Smith


[email protected]
João Fernandes


[email protected]
Johannes Van der Berg


[email protected]
John Gordon


[email protected]
Julia Barnett


[email protected]
Kara Nielsen


[email protected]
Leonie Köhler


[email protected]
Lucas Mancini


[email protected]
Luís Gonçalves


[email protected]
Luis Rojas


[email protected]
Manoj Pareek


[email protected]
Marc Dubois


[email protected]
Madalena Sampaio


[email protected]
Michelle Brooks


[email protected]
Mark Philips


[email protected]
Niklas Schröder


[email protected]
Patrick Gray


[email protected]
Phil Hughes


stanisław.wó[email protected]
Stanisław Wójcik


[email protected]
Steve Murray


[email protected]
Terhi Hämäläinen


[email protected]
Tim Goyer


[email protected]
Victor Stevens


[email protected]
Wyatt Girard


It's possible for a customer to buy several Jazz music items. So the customer details will be returned as many times as they bought Jazz music. We've used the DISTINCT keyword to return only one detail for this type of customer.
19. Write a query to return the top 5 spenders on Jazz and Rock
Answer
We'll continue to work with the chinook.db. We need to join the customer and genre tables and filter by the type of genre. We'll be returning the customer full_name, and the amount_spent. To get the amount spent, we'll look at the invoice_line table for the unit_price and quantity purchased.  
A single customer can buy music from a genre more than once. Their information will apply multiple times. You need to GROUP BY the customer full_name and SUM the values from the product of unit_price and quantity to get the total amount spent on that genre:

     * sqlite:///chinook.db
    Done.


full_name
amount_spent


Fernanda Ramos
15.84


Enrique Muñoz
15.84


Hannah Schneider
14.85


Astrid Gruber
14.85


Kara Nielsen
3.96



     * sqlite:///chinook.db
    Done.


full_name
amount_spent


Helena Holý
75.24


François Tremblay
74.25


Luís Gonçalves
71.28


Hugh O'Reilly
71.28


João Fernandes
67.32


You can easily get the amount_spent values wrong if you use the total column in the invoice table. This is because a customer can buy songs from more than one genre on the same invoice, and the total column will contain the full amount spent, not the amount spent on a particular genre. 
This is demonstrated in the query below. Ellie Sullivan bought 10 songs from different genres on 2017-04-16. The total amount spent on all the songs is 9.9, which is different from the amount spent on a particular genre.

     * sqlite:///chinook.db
    Done.


full_name
name
unit_price
quantity
total


Ellie Sullivan
Metal
0.99
1
9.9


Ellie Sullivan
Rock
0.99
1
9.9


Ellie Sullivan
Rock
0.99
1
9.9


Ellie Sullivan
Rock
0.99
1
9.9


Ellie Sullivan
Rock
0.99
1
9.9


Ellie Sullivan
Rock
0.99
1
9.9


Ellie Sullivan
Rock
0.99
1
9.9


Ellie Sullivan
Rock
0.99
1
9.9


Ellie Sullivan
Metal
0.99
1
9.9


Ellie Sullivan
Alternative & Punk
0.99
1
9.9


20. Simplify the query to return the top 5 spenders on Jazz with CTE
CTE stands for Common Table Expression. It enables us to simplify complex queries by deconstructing them into simple blocks. We initiate a CTE with the WITH clause. The SQL query that answers the previous question is a bit complex. We can make the query more readable by performing most of the join operations with CTE. 

     * sqlite:///chinook.db
    Done.


full_name
amount_spent


Fernanda Ramos
15.84


Enrique Muñoz
15.84


Hannah Schneider
14.85


Astrid Gruber
14.85


Kara Nielsen
3.96


Close connection to chinook.db:

Takeaway
In this tutorial, we discussed the top 20 SQL JOINs interview questions and provided a step-by-step guide to their answers. If you aspire to become a business analyst, data analyst, or data scientist, knowledge of SQL is one of the leading requirements for your role. No two ways about it: you must learn SQL programming.
As you work with relational databases, one of the most common SQL operations you'll perform is joining tables. This is why SQL JOIN questions are so popular with interviewers. At your SQL interviews, you'll get several questions on SQL JOINs to test your knowledge of how they work. We've provided answers to some of the most common questions you may be asked.
If you haven't learned the basics of SQL yet, this tutorial may not be helpful. Learn how to explore, query, join, and filter data from relational databases in our SQL Fundamentals Skill Path. You'll master the SQL syntaxes and basic commands that you'll need to start working with relational databases. On our interactive learning platform, you get to practice writing your own SQL queries. 
We understand that porfolio projects are very important, and they help showcase your SQL programming and data analysis skills to potential recruiters.  You get to complete real-life SQL projects on the SQL Fundamentals Skill Path. We've also curated 10 exiciting SQL project ideas to help you get more practice and projects in your SQL portfolio.
                    

                
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