SQL, or Structured Query Language, is regarded as the core of relational databases, which provides aid in handling data. It offers extensive features, including Injection, Triggers, Joins, Hosting, etc.
Joins are one of the most important concepts of SQL. In this article, we will give you informative insights into SQL JOINs and also look into the different types of SQL Joins. Please keep reading until the end of the article to know more about SQL Joins in detail. Let’s begin!
SQL is a programming language that is used for storing and processing information within a relational database. A relational database stores information in a tabular format representing different data attributes and relationships between the data values.
You can also use SQL statements for storing, updating, removing, searching, and retrieving information from the database. Besides, it can also be used for maintaining and optimising database performance.
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What are JOINs in SQL?
JOINs in SQL are typical commands that combine rows from different tables based on related columns between those tables. Users predominantly use this command while extracting data from tables that share many-to-many relationships.
Importance of SQL JOINs
SQL JOINs enable you to combine data from tables based on related columns. The importance of SQL JOINs can be stated as follows:
Data integrity
Data retrieval
Optimised performance
Flexible data query
Business intelligence
Query optimisation, etc.
Different Types of SQL JOINs
The SQL server primarily supports four distinctive variants of JOINs. Each JOIN query in SQL defines how two or more tables are related within a query.
Here we have enlisted the various types of SQL JOIN support within the SQL Server.
Inner Joins in SQL
Outer Joins in SQL
Cross Joins in SQL
Right Joins in SQL
Left Joins in SQL
Full-outer Joins in SQL
Inner JOINs in SQL
This type of JOIN in SQL returns all records from multiple tables that satisfy the specified JOIN condition. It is considered the simplest and the most popular form of JOIN in SQL that assumes a default JOIN. Omitting the INNER keyword from the JOIN query will fetch you a similar output.
Functions of Inner JOINs in SQL
This type of JOIN in SQL combines rows from two or more tables based on an entirely matching condition. It only returns the rows that have matching values in both tables.
Syntax and How to Use Them
SELECT columns
FROM table1
INNER JOIN table2 ON condition1
INNER JOIN table3 ON condition2
Examples
Firstly, we will create two tables “Student” and “Fee” using the following statement:
CREATE TABLE Student (
id int PRIMARY KEY IDENTITY,
admission_no varchar(45) NOT NULL,
first_name varchar(45) NOT NULL,
last_name varchar(45) NOT NULL,
age int,
city varchar(25) NOT NULL
);
CREATE TABLE Fee (
admission_no varchar(45) NOT NULL,
course varchar(45) NOT NULL,
amount_paid int,
);
Now, we will incorporate a few records into these tables by using these statements:
INSERT INTO Student (admission_no, first_name, last_name, age, city)
VALUES (3354,'Luisa', 'Evans', 13, 'Texas'),
(2135, 'Paul', 'Ward', 15, 'Alaska'),
(4321, 'Peter', 'Bennett', 14, 'California'),
(4213,'Carlos', 'Patterson', 17, 'New York'),
(5112, 'Rose', 'Huges', 16, 'Florida'),
(6113, 'Marielia', 'Simmons', 15, 'Arizona'),
(7555,'Antonio', 'Butler', 14, 'New York'),
(8345, 'Diego', 'Cox', 13, 'California');
INSERT INTO Fee (admission_no, course, amount_paid)
VALUES (3354,'Java', 20000),
(7555, 'Android', 22000),
(4321, 'Python', 18000),
(8345,'SQL', 15000),
(5112, 'Machine Learning', 30000);
Next, you need to execute the SELECT statement for verifying the records:
Table: Student
ID
Admission_No
First_Name
Last_Name
Age
City
1
3354
Luisa
Evans
13
Texas
2
2135
Paul
Ward
15
Alaska
3
4321
Peter
Bennett
14
California
4
4213
Carlos
Patterson
17
New York
5
5112
Rose
Huges
16
Florida
6
6113
Marielia
Simmons
15
Arizona
7
7555
Antonio
Butler
14
New York
8
8345
Diego
Cox
13
California
Table: Fee
Admission_no
Course
Amount_Paid
3345
Java
20000
7555
Android
22000
4321
Python
18000
8345
SQL
15000
5112
Machine Learning
30000
Best Practices and Common Pitfalls
Using proper indexing
Using aliases for readability
Avoiding excessive tables, etc.
Some of the common pitfalls that you’re likely to encounter while using this JOIN in SQL are:
Incorrect JOIN conditions
Data skewness
Performance issues, etc.
Outer JOINs
Outer JOINs in SQL Server return all records from both tables that genuinely satisfies the JOIN condition. Simply put, thisSQL JOIN will return the matching record and all the unmatched rows from all tables.
Explanation of Outer JOINs and Their Function
Outer JOINs in SQL return all rows from one table and matching rows from another. Furthermore, it also includes non-matching rows from one or both tables.
Syntax for Left, Right, and Full Outer SQL JOINs and How to Use Them
The following syntax illustrates the use of left outer JOIN in SQL Server:
SELECT column_lists
FROM table1
LEFT [OUTER] JOIN table2
ON table1.column = table2.column;
The following syntax illustrates the use of right outer JOIN in SQL Server:
SELECT column_lists
FROM table1
RIGHT [OUTER] JOIN table2
ON table1.column = table2.column;
The following syntax illustrates the use of full outer JOIN in SQL Server:
SELECT column_lists
FROM table1
FULL [OUTER] JOIN table2
ON table1.column = table2.column;
Examples of Outer JOINs in Action
We can seamlessly demonstrate the LEFT OUTER JOIN using the following command:
SELECT Student.admission_no, Student.first_name, Student.last_name, Fee.course, Fee.amount_paid
FROM Student
LEFT OUTER JOIN Fee
ON Student.admission_no = Fee.admission_no;
Here’s howe you can use the RIGHT OUTER JOIN to get records from both tables:
SELECT Student.admission_no, Student.first_name, Student.last_name, Fee.course, Fee.amount_paid
FROM Student
RIGHT OUTER JOIN Fee
ON Student.admission_no = Fee.admission_no;
This example clearly explains the techniques of using the FULL OUTER JOIN to get records from both tables:
SELECT Student.admission_no, Student.first_name, Student.last_name, Fee.course, Fee.amount_paid
FROM Student
FULL OUTER JOIN Fee
ON Student.admission_no = Fee.admission_no;
Best Practices and Common Pitfalls When Using Outer JOINs
Some of the best practices for using Outer JOINs include:
Using appropriate JOIN syntax
Defining the primary purpose of JOIN
Understanding the order of operations
Testing the results with sample data
Some of the most common pitfalls that you’re likely to encounter are:
Using inappropriate JOIN syntax
Understanding the differences between various types of outer JOINs
Not accounting for NULL values
Cross JOINs
The cross JOINs in SQL combine all the possibilities of different tables, returning results from each row of every contributing table.
Explanation of Cross JOINs and Their Function
Cross JOIN is a join in SQL that produces a Cartesian product of two tables. It results in all possible combinations of the rows and doesn’t require any standard columns between the tables.
Syntax for Cross JOINs in SQL and How to Use Them
SELECT column_lists
FROM table1
CROSS JOIN table2;
Examples of Cross JOINs in Action
Here’s how we can demonstrate the CROSS JOIN with the following command:
SELECT Student.admission_no, Student.first_name, Student.last_name, Fee.course, Fee.amount_paid
FROM Student
CROSS JOIN Fee
WHERE Student.admission_no = Fee.admission_no;
Best Practices and Common Pitfalls When Using Cross JOINs
The best practices for using cross JOINs include:
Using proper filtering only to limit the size of the result set, understanding data and the primary purpose of SQL JOIN
Testing all results with the help of sample data
Some of the common pitfalls that you might encounter are:
An extensive number of rows
Producing unintended combinations
Noticeable performance issues
Comparison of Different Types of JOINs in SQL
SQL JOIN Type
Description
INNER JOIN
Returns only the matching rows from both tables based on a specified condition
LEFT JOIN
Returns all rows from the left table and matching rows from the right table, or NULL values if there is no match
RIGHT JOIN
Returns all rows from the right table and matching rows from the left table, or NULL values if there is no match
FULL OUTER JOIN
Returns all rows from both tables, including NULL values for non-matching rows
CROSS JOIN
Returns the Cartesian product of both tables, producing all possible combinations of their rows
Choosing the Right Type of SQL JOIN for Your Data Needs
Consider the relationship between the data and tables you wish to retrieve to pick out the right JOIN type. You can use the INNER JOIN to retrieve matching data and LEFT or RIGHT JOIN to include non-matching data. Besides, you can also use the FULL OUTER JOIN to include all data and CROSS JOIN to combine all possible combinations.
Conclusion
In this article we have what SQL joins and, the major types of joins in SQL. To sum up, SQL JOINs enable you to combine data from multiple tables. You must choose the appropriate SQL JOIN type based on the relationship between tables and data requirements.
The primary purpose of using JOINs in SQL is to access data from multiple tables according to their logical relationships. SQL JOINs are used to fetch data from database tables and represent the result dataset as a separate table.
Which type of SQL JOIN delivers the best performance?
There are various types of joins in SQL. The LEFT OUTER JOINs deliver faster and better performance in most cases.
How can I reduce the number of JOINS in SQL?
If you wish to remove JOINs in SQL, you need to incorporate the technique of JOIN elimination. The SQL Server query optimiser mostly uses this unique technique to create efficient query plans. SQL JOIN elimination usually occurs when SQL Server can establish equality by using trusted database constraints to eliminate unnecessary JOINs in SQL.
Can I join more than three tables in SQL?
You can easily use multiple JOIN statements at once to join more than a single table simultaneously. However, you must add a second INNER JOIN statement and a second ON statement. This will help in indicating the third table and also the second relationship.
How can I find duplicate records in SQL?
You can use the keywords COUNT and GROUP BY to find duplicate records in SQL. This command will help you obtain all the duplicate records while using INNER JOIN along with COUNT and GROUP BY.
