Understanding SQL Joins – All You Need To Know About SQL Joins

Structured Query Language aka SQL is the core of relational databases with the help of which we can handle data. It provides us with various features such as Triggers, Injection, Hosting and, Joins is just one of the most important concept to master in SQL. In this article on SQL Joins, I will discuss the various types of Joins used in SQL.

The following topics will be covered in this article

• • What are Joins?
  • How many types of Joins are there in SQL? 
  • How do I know which join to use in SQL
    • INNER JOIN
    • FULL JOIN
    • LEFT JOIN
    • RIGHT JOIN
  • Most Common Questions asked about Joins

What are Joins?

JOINS in SQL are commands which are used to combine rows from two or more tables, based on a related column between those tables.  There are predominantly used when a user is trying to extract data from tables which have one-to-many or many-to-many relationships between them.

Now, that you know what joins mean, let us next learn the different types of joins.

How many types of Joins are there in SQL?

There are mainly four types of joins that you need to understand. They are:

• INNER JOIN
• FULL JOIN
• LEFT JOIN
• RIGHT JOIN

You can refer to the below image.

How do I know which join to use in SQL?

Let us look into each one of them. For your better understanding of this concept, I will be considering the following three tables to show you how to perform the Join operations on such tables.

Employee Table:

EmpID	EmpFname	EmpLname	Age	EmailID	PhoneNo	Address
1	Vardhan	Kumar	22	vardy@abc.com	9876543210	Delhi
2	Himani	Sharma	32	himani@abc.com	9977554422	Mumbai
3	Aayushi	Shreshth	24	aayushi@abc.com	9977555121	Kolkata
4	Hemanth	Sharma	25	hemanth@abc.com	9876545666	Bengaluru
5	Swatee	Kapoor	26	swatee@abc.com	9544567777	Hyderabad

Project Table:

ProjectID	EmpID	ClientID	ProjectName	ProjectStartDate
111	1	3	Project1	2019-04-21
222	2	1	Project2	2019-02-12
333	3	5	Project3	2019-01-10
444	3	2	Project4	2019-04-16
555	5	4	Project5	2019-05-23
666	9	1	Project6	2019-01-12
777	7	2	Project7	2019-07-25
888	8	3	Project8	2019-08-20

Client Table:

ClientID	ClientFname	ClientLname	Age	ClientEmailID	PhoneNo	Address	EmpID
1	Susan	Smith	30	susan@adn.com	9765411231	Kolkata	3
2	Mois	Ali	27	mois@jsq.com	9876543561	Kolkata	3
3	Soma	Paul	22	soma@wja.com	9966332211	Delhi	1
4	Zainab	Daginawala	40	zainab@qkq.com	9955884422	Hyderabad	5
5	Bhaskar	Reddy	32	bhaskar@xyz.com	9636963269	Mumbai	2

INNER JOIN

This type of join returns those records which have matching values in both tables. So, if you perform an INNER join operation between the Employee table and the Projects table, all the tuples which have matching values in both the tables will be given as output.

Syntax:

SELECT Table1.Column1,Table1.Column2,Table2.Column1,....
FROM Table1
INNER JOIN Table2
ON Table1.MatchingColumnName = Table2.MatchingColumnName;

Example:

SELECT Employee.EmpID, Employee.EmpFname, Employee.EmpLname, Projects.ProjectID, Projects.ProjectName
FROM Employee
INNER JOIN Projects ON Employee.EmpID=Projects.EmpID;

Output:

EmpID	EmpFname	EmpLname	ProjectID	ProjectName
1	Vardhan	Kumar	111	Project1
2	Himani	Sharma	222	Project2
3	Aayushi	Shreshth	333	Project3
3	Aayushi	Shreshth	444	Project4
5	Swatee	Kapoor	555	Project5

FULL JOIN

Full Join or the Full Outer Join returns all those records which either have a match in the left(Table1) or the right(Table2) table.

Syntax:

SELECT Table1.Column1,Table1.Column2,Table2.Column1,....
FROM Table1
FULL JOIN Table2
ON Table1.MatchingColumnName = Table2.MatchingColumnName;

Example:

SELECT Employee.EmpFname, Employee.EmpLname, Projects.ProjectID
FROM Employee
FULL JOIN Projects
ON Employee.EmpID = Projects.EmpID;

Output:

EmpFname	EmpLname	ProjectID
Vardhan	Kumar	111
Himani	Sharma	222
Aayushi	Shreshth	333
Aayushi	Shreshth	444
Hemanth	Sharma	NULL
Swatee	Kapoor	555
NULL	NULL	666
NULL	NULL	777
NULL	NULL	888

LEFT JOIN

The LEFT JOIN or the LEFT OUTER JOIN  returns all the records from the left table and also those records which satisfy a condition from the right table. Also, for the records having no matching values in the right table, the output or the result-set will contain the NULL values.

Syntax:

SELECT Table1.Column1,Table1.Column2,Table2.Column1,....
FROM Table1
LEFT JOIN Table2
ON Table1.MatchingColumnName = Table2.MatchingColumnName;

Example:

SELECT Employee.EmpFname, Employee.EmpLname, Projects.ProjectID, Projects.ProjectName
FROM Employee
LEFT JOIN
ON Employee.EmpID = Projects.EmpID ;

Output:

EmpFname	EmpLname	ProjectID	ProjectName
Vardhan	Kumar	111	Project1
Himani	Sharma	222	Project2
Aayushi	Shreshth	333	Project3
Aayushi	Shreshth	444	Project4
Swatee	Kapoor	555	Project5
Hemanth	Sharma	NULL	NULL

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RIGHT JOIN

The RIGHT JOIN or the RIGHT OUTER JOIN  returns all the records from the right table and also those records which satisfy a condition from the left table. Also, for the records having no matching values in the left table, the output or the result-set will contain the NULL values.

Syntax:

SELECT Table1.Column1,Table1.Column2,Table2.Column1,....
FROM Table1
RIGHT JOIN Table2
ON Table1.MatchingColumnName = Table2.MatchingColumnName;

Example:

SELECT Employee.EmpFname, Employee.EmpLname, Projects.ProjectID, Projects.ProjectName
FROM Employee
RIGHT JOIN
ON Employee.EmpID = Projects.EmpID;

Output:

EmpFname	EmpLname	ProjectID	ProjectName
Vardhan	Kumar	111	Project1
Himani	Sharma	222	Project2
Aayushi	Shreshth	333	Project3
Aayushi	Shreshth	444	Project4
Swatee	Kapoor	555	Project5
NULL	NULL	666	Project6
NULL	NULL	777	Project7
NULL	NULL	888	Project8

Now, let us move forward with our next section in this article i.e. the top questions asked about SQL Joins in your interviews.

Most Common Questions asked about Joins

Question 1: What is a Natural Join and in which situations is a natural join used?

Solution:

A Natural Join is also a Join operation that is used to give you an output based on the columns in both the tables between which, this join operation must be implemented. To understand the situations n which natural join is used, you need to understand the difference between Natural Join and Inner Join.

The main difference the Natural Join and the Inner Join relies on the number of columns returned. Refer below for example.

Now, if you apply INNER JOIN on these 2 tables, you will see an output as below:

If you apply NATURAL JOIN, on the above two tables, the output will be as below:

From the above example, you can clearly see that the number of columns returned from the Inner Join is more than that of the number of columns returned from Natural Join. So, if you wish to get an output, with less number of columns, then you can use Natural Join

Question 2: How to map many-to-many relationships using joins?

Solution:

To map many to many relationships using joins, you need to use two JOIN statements.

For example, if we have three tables(Employees, Projects and Technologies), and let us assume that each employee is working on a single project. So, one project cannot be assigned to more than one employee. So, this is basically, a one-to-many relationship.

Now, similarly, if you consider that, a project can be based on multiple technologies, and any technology can be used in multiple projects, then this kind of relationship is a many-to-many relationship.

To use joins for such relationships, you need to structure your database with 2 foreign keys. So, to do that, you have to create the following 3 tables:

• Projects
• Technologies
• projects_to_technologies

The project_to_technologies table holds the combinations of project-technology in every row. This table maps the items on the projects table to the items on the technologies table so that multiple projects can be assigned to one or more technologies.

Once the tables are created, use the following two JOIN statements to link all the above tables together:

• projects_to_technologies to projects
• proejcts_to-technologies to technologies

Question 3: What is a Hash Join?

Solution:

Hash joins are also a type of joins which are used to join large tables or in an instance where the user wants most of the joined table rows.

The Hash Join algorithm is a two-step algorithm. Refer below for the steps:

• Build phase: Create an in-memory hash index on the left side input
• Probe phase: Go through the right side input, each row at a time to find the matches using the index created in the above step.

Question 4: What is Self & Cross Join?

Solution:

Self Join

SELF JOIN in other words is a join of a table to itself. This implies that each row in a table is joined with itself.

Cross Join

The CROSS JOIN is a type of join in which a join clause is applied to each row of a table to every row of the other table. Also, when the WHERE condition is used, this type of JOIN behaves as an INNER JOIN, and when the WHERE condition is not present, it behaves like a CARTESIAN product.

Question 5: Can you JOIN 3 tables in SQL?

Solution:

Yes. To perform a JOIN operation on 3 tables, you need to use 2 JOIN statements. You can refer to the second question for an understanding of how to join 3 tables with an example.

Now that you know SQL Joins, I’m sure you’re curious to learn more about SQL. Here’s a list of articles that you can refer to:

1. SQL Commands
2. SQL Data Types
3. Spark SQL
4. SQL Interview Questions
5. What is MYSQL?

By this, I come to the end of this article on SQL Joins. I hope you enjoyed reading this article on SQL Joins. We have seen the different commands that will help you write queries and play around with your databases. If you wish to learn more about MySQL and get to know this open source relational database, then check out our MySQL DBA Certification Training which comes with instructor-led live training and real-life project experience. This SQL training will help you understand MySQL in depth and help you achieve mastery over the subject.

Got a question for us? Please mention it in the comments section of ”SQL Joins” and I will get back to you.