Explicit Join vs Implicit Join in SQL

Explicit and Implicit Joins are two important methods in SQL for combining data from multiple tables. Combining data from multiple tables is a common task when working with SQL databases. SQL provides these two key methods to achieve this, helping retrieve related data efficiently and ensuring accurate results. Both allow you to retrieve data from multiple tables, but they differ in how they specify the join conditions. Explicit Joins offer greater flexibility and control over how data is combined. On the other hand, Implicit Join is a more traditional approach. In this blog, you will explore Explicit and Implicit Joins in SQL, understand their key differences, and see practical examples of when and how to use each method.

Table of Contents:

• Why do we need JOINS?
• What are Explicit Joins?
  • INNER JOIN
  • LEFT JOIN
  • RIGHT JOIN
  • FULL OUTER JOIN
• Features of Explicit Join
• Why Explicit Joins Are Preferred?
• What are Implicit Joins?
  • Features of Implicit Join
  • Drawbacks of Implicit Join
  • Why Does Implicit Joins Affect the Performance?
• Explicit vs Implicit Join
• Best Practices
• Conclusion

Why do we need JOINS?

In database systems, we usually need to relate information to each other that is stored in different tables. For example, in the Intellipaat database, we might have a list of students and a list of courses. To find out which students are enrolled in which course, we need to connect these two tables. This is where SQL joins come into play. Let’s walk through a practical example of how to do this.

We’ll create three tables: Students, Courses, and Student Course table. Each student can be enrolled in multiple courses. We’ll use a junction table to link them and then perform a join, which can be used for all the examples below.

CREATE TABLE Students (
    StudentID INT PRIMARY KEY,
    StudentName VARCHAR(50)
);
-- Insert Data into Students' Table
INSERT INTO Students (StudentID, StudentName) VALUES
(1, 'Alice'),
(2, ‘Mathew’'),
(3, 'Edwin’');
-- To display the Output
Select * from Students;

This is what the Students table looks likeafter creating and inserting the values.

Let’s create one more table for a better understanding of JOINS

-- Create Courses Table
CREATE TABLE Courses (
    CourseID INT PRIMARY KEY,
    CourseName VARCHAR(50)
);
-- Insert Data into Courses Table
INSERT INTO Courses (CourseID, CourseName) VALUES
(101, 'Math'),
(102, 'Science');
-- To display the Course table
Select * from Courses;

This is how the Courses table looks after creating and inserting the values.

Let’s create a Student Course table and insert some values into it

-- Create the StudentCourses 
CREATE TABLE StudentCourses (
    StudentID INT,
    CourseID INT,
    PRIMARY KEY (StudentID, CourseID),
    FOREIGN KEY (StudentID) REFERENCES Students(StudentID),
    FOREIGN KEY (CourseID) REFERENCES Courses(CourseID)
);
--Insert Data into StudentCourses Junction Table

INSERT INTO StudentCourses (StudentID, CourseID) VALUES
(1, 101), -- Alex takes Data Science
(1, 102), -- Alex takes Cloud
(2, 102), -- Mathew takes Cloud
(3, 101); -- Edwin takes Data Science
--Let's look at what the  StudentCourses  Table Looks like in SQL.
Select * from StudentCourses Table;

This is how the StudentCourses table after creating and inserting the values

Example:

-- Using an INNER JOIN to retrieve student  course information
SELECT Students.StudentName, Courses.CourseName
FROM Students
INNER JOIN StudentCourses ON Students.StudentID = StudentCourses.StudentID
INNER JOIN Courses ON StudentCourses.CourseID = Courses.CourseID;

Output:

Explanation: Here, the INNER JOIN combines rows from the Students, StudentCourses, and Courses table based on matching StudentID and CourseID values and the result shows which student is enrolled in which course.

What are Explicit Joins?

An Explicit Join in SQL is used to merge two or more rows based on a common attribute. It’s called “explicit” because you specify the type of join you want to use (e.g., INNER JOIN, LEFT JOIN) and the condition for matching rows with the help of the ON Clause. This contrasts with implicit joins, also known as comma joins, which are generally considered outdated and less readable.

Syntax:

SELECT columns
FROM table1
JOIN table2 ON table1.column = table2.column;

There are mainly four types of Explicit Joins:

1. INNER JOIN

The INNER JOIN returns rows only when there is a match in the specific columns of both tables. It filters out unmatched rows to ensure you see only related data that exists across your table. It’s the most common join which is used to retrieve data.

Syntax:

SELECT column1, column2, ...
FROM table1
INNER JOIN table2 ON table1.matching_column = table2.matching_column;

2. LEFT JOIN

The LEFT JOIN is used to return all records from the left-hand side table and matching records from the right-hand side table, and if there is no match in the right-hand table, then null values are returned.

Syntax:

SELECT column1, column2, ...

FROM table1

LEFT JOIN table2 ON table1.matching_column = table2.matching_column;

3. RIGHT JOIN

The RIGHT JOIN returns all rows from the right-hand table and matching rows from the left-hand table. If there is no match in the left-hand table, then null values are returned. This makes sure all data from the right table is included

Syntax:

SELECT column1, column2, ...

FROM table1

RIGHT JOIN table2 ON table1.matching_column = table2.matching_column;

4. FULL OUTER JOIN

The FULL OUTER JOIN combines all records from two tables, and if there is a match between the two tables, then it combines the row and if there are no matches, null values are returned.

Syntax:

SELECT column1, column2, ...

FROM table1

FULL OUTER JOIN table2 ON table1.matching_column = table2.matching_column;

Example:

-- Explicit Join Example: Using data from the Student and StudentCourses table for a better understanding

SELECT Students.StudentName, Courses.CourseName

FROM Students

INNER JOIN StudentCourses ON Students.StudentID = StudentCourses.StudentID

INNER JOIN Courses ON Courses.CourseID = StudentCourses.CourseID;

Output:

Explanation: Here, this query retrieves the names of students and the courses they are enrolled in. It joins three tables-Students, StudentCourse, and Courses using StudentID and CourseID.

Features of Explicit Join

• Readable: Explicit Joins make join conditions easy, which makes code readable and understandable.
• Maintainable: They simplify modifications and debugging by separating join logic from other elements in the query.
• Flexible: Explicit Joins support all types of joins (INNER, LEFT, RIGHT, FULL OUTER), providing greater flexibility.
• Reduced Uncertainty: They reduce the chance of accidentally creating Cartesian products.

Why Explicit Joins Are Preferred?

• Increased Query Control: Explicit Join offers finer control over the join operation, which allows more complex queries.
• Standard Syntax: Explicit Joins have standard SQL synta,x which ensures compatibility across different database systems.
• Improves debugging: When errors occur, explicit join helps to point out the issue quickly.
• Separate Join type and condition: They separate the join type(INNER, LEFT, RIGHT, FULL) from the specific join criteria, which brings clarity about joins
• Reduced Risk of Accidental Cartesian Products: By explicitly defining join conditions, chances of unintentional incorrect results are reduced.

What are Implicit Joins?

In SQL, Implicit Joins are also known as comma joins, which represent an older style of combining data from multiple tables. While they can achieve similar results to explicit joins, they are generally discouraged in modern SQL development due to potential clarity. Implicit Joins are an Older and Less Clear Approach. They use commas to separate tables in the FROM Clause and specify the Join Condition in the WHERE clause.

Syntax:

SELECT columns

FROM table1, table2

WHERE table1.column = table2.column;

Example:

--Implicit Join Example: Using data from the Student and StudentCourses table for a better understanding

SELECT Students.StudentName, Courses.CourseName

FROM Students, Courses, StudentCourses

WHERE Students.StudentID = StudentCourses.StudentID

AND Courses.CourseID = StudentCourses.CourseID;

Output:

Explanation: Here, Implicit Joins combine tables by listing them in the FROM clause and specifying JOIN conditions in the WHERE clause. Explicit joins with INNER JOIN and ON are preferred for a clear understanding

Features of Implicit Join

• Reduced Readability: Mixing join and filter conditions in the WHERE clause makes the query harder to read.
• Join Condition in WHERE CLAUSE: Join criteria are specified within the WHERE clause, and they are mixed with filtering conditions.
• Comma-separated tables in FROM Clause: Tables to be joined are listed in the FROM clause, separated by commas.
• Older and lesser Approaches: It represents an older, less standardized way of writing joins.
• Risk of Accidental Joins: If join conditions are incorrect, then there is an increased risk of accidental cross-join.
• Reduced Readability: Mixing join and filter conditions in the WHERE clause makes the query harder to read.

Drawbacks of Implicit Join

• Harder to Read: The Tables are listed in one place, and how they connect is hidden somewhere else. It’s just hard and confusing to understand how implicit joins work, especially when you have lots of tables.
• Increased Risk of Errors: If you forget to tell the database how the tables are related, it just smashes every row from one table with every row from the other, creating a huge mess.
• Outdated Practice: Implicit joins are an older style of writing SQL queries. While they were commonly used in earlier versions of SQL. Now they’ve become less popular, as explicit joins are considered clearer and more maintainable.

Why Do Implicit Joins Affect the Performance?

• Confused Database: The Database has a harder time figuring out the best way to run the query, which can lead to slow execution.
• Unnecessary Results: If you forget to connect the tables correctly, then you’ll get a massive and useless result that takes forever to process.
• Extra Work: Even if the database gets it right, it has to work harder to understand what you meant, which can slow things down.

Explicit vs Implicit Join

Best Practices

• Always use Explicit Joins: Explicit joins make your queries more powerful, and they are easier to understand.
• Format your code for readability: Proper spacing and indentation reduce errors and increase debugging speed.
• Use aliases for table and column names: Short names improve readability when you are working with multiple tables.
• Test your queries: Before deploying any query, test it on the staging database to verify its correctness and performance.

Conclusion

Explicit Joins are a clear, reliable, and modern way to combine data in SQL. By using them, you can write code that’s easier to understand, maintain, and debug. Avoid implicit joins to minimize errors and potential performance issues. Use the explicit join to make your database work faster and easier to manage.

To learn more about SQL functions, check out this SQL course and also explore SQL Interview Questions prepared by industry experts.

Explicit vs Implicit Join – FAQs