Explicit vs Implicit Joins in SQLite

When working with SQLite databases, users often need to retrieve data from multiple tables. Joins are used to combine data from these tables based on a common column. SQLite supports two types of joins which are explicit joins and implicit joins. In this article, We’ll learn about Explicit vs implicit joins in SQLite along with their differences and some examples and so on.

What is Join in SQLite?

Joins in SQLite are used to combine data from multiple tables based on a related column between them. The Joins further classified in two ways which specify the relationships between tables in a query which are:

1. Explicit join
2. Implicit join

Joins in SQLite

What are Explicit Joins?

An explicit join refers to the use of the JOIN keyword to explicitly define the relationship between tables in a query. In explicit joins the tables to be joined are listed in the FROM Clause and the relationship between them is specified using the JOIN keyword along with the ON keyword to define the join condition.

Consider we have the following three tables called EMPLOYEES, DEPARTMENT, and EMPLOYEE_DEPARTMENT. Here EMPLOYEES consists of id, first_name, last_name, and DEPARTMENT consists of id, Nname and EMPLOYEE_DEPARTMENT consist of employee_id, department_id as Columns.

Database(Explicit Join)

Example 1

Let’s Retrieve the first name and last name of employees along with the name of their departments.

SELECT employees.first_name, employees.last_name, departments.name
FROM employees
INNER JOIN employee_department ON employees.id = employee_department.employee_id
INNER JOIN departments ON employee_department.department_id = departments.id;

Output:

Explicit join (Example-1 output)

Explanation: This query uses two inner joins to retrieve data from three tables which are employees, employee_department and departments. It selects the first name and last name of employees along with the name of their departments. The INNER JOIN keyword is used to return only the rows where there is at least one match in both tables.

Example 2

Let’s Retrieve the first name and last name of employees along with the name of their departments, including employees who are not currently assigned to any department.

SELECT employees.first_name, employees.last_name, departments.name
FROM employees
LEFT JOIN employee_department ON employees.id = employee_department.employee_id
LEFT JOIN departments ON employee_department.department_id = departments.id;

Output:

Explicit join (Example-2 output)

Explanation: In the above query we uses two left joins to retrieve data from three tables which are “employees,” “employee_department,” and “departments.” It selects the first name and last name of employees along with the name of their departments. The LEFT JOIN keyword is used to ensure that all records from the “employees” table are included in the result even if there is no matching record in the “employee_department” or “departments” tables.

Example 3

Let’s Retrieve the first name and last name of all employees along with the name of all departments, creating a combination of every employee with every department, regardless of their actual assignments.

SELECT employees.first_name, employees.last_name, departments.name
FROM employees
CROSS JOIN departments;

Output:

Example-3 output

Explanation: In the above Query, We performs a cross join between the “employees” and “departments” tables, combining every row from the “employees” table with every row from the “departments” table. This results in a Cartesian product where each row in the output represents a combination of an employee’s first name and last name with a department’s name.

What is Implicit Joins?

Implicit joins in SQLite refer to joining tables in a query using the WHERE clause without explicitly using the JOIN keyword. This method of joining tables is considered implicit because the join condition is implied by the WHERE clause, rather than explicitly stated in a JOIN clause. Let’s understand through the below examples.

Consider the following two tables called STUDENTS and COURSES. Here STUDENTS consists of id,first_name, last_name and COURSES consist of id, course_name as Columns.

Implicit Joins( Database)

Example 1

Let’s Retrieve the first name and last name of students along with the name of the course they are enrolled in, based on matching IDs between the students and courses tables.

SELECT students.first_name, students.last_name, courses.course_name
FROM students, courses
WHERE students.id = courses.id;

Output:

Implicit Joins( Example-1 output)

Explanation: In the above query, We selects the first name and last name of students along with the name of the course they are enrolled in, based on matching IDs between the students and courses tables. It retrieves data from the “students” and “courses” tables where the ID of a student matches the ID of a course.

Example 2

Let’s Retrieve the first name and last name of students along with the name of the course they are enrolled in, without any specific condition for matching IDs between the students and courses tables.

SELECT students.first_name, students.last_name, courses.course_name
FROM students, courses;

Output:

Implicit Joins( Example-2 output)

Explanation: In the above query, We selects the first name and last name of students along with the name of the courses they are enrolled in. It retrieves data from the “students” table and the “courses” table, combining every row from the “students” table with every row from the “courses” table which effectively creating a Cartesian product of the two tables

Explicit vs Implicit Joins

Aspect	Explicit Joins	Implicit Joins
Syntax	Uses JOIN and ON keywords to specify the join condition.	Tables are listed in the FROM clause, join conditions are specified in the WHERE clause.
Clarity	Offers clearer syntax, explicitly indicating the relationship between tables.	May be less clear, as the join conditions are embedded in the WHERE clause.
Readability	Generally considered more readable and maintainable due to explicit syntax.	May be less readable, especially in complex queries with multiple join conditions.
Flexibility	Provides more control over join conditions, allowing for different types of joins (e.g., INNER, LEFT)	Limited flexibility, as join conditions are limited to conditions in the WHERE clause.
Performance	Generally performs better, as the query optimizer can better optimize explicit joins.	May have performance implications, as the query optimizer may not be able to optimize implicit joins as effectively.
Debugging and Troubleshooting	Easier to debug, as join conditions are explicitly stated and separate from other conditions.	May be harder to debug, especially in complex queries, as join conditions are mixed with other conditions.

Conclusion

In conclusion, explicit joins in SQLite offer clearer syntax and greater control over join conditions, enhancing readability, maintainability, and debugging efficiency. With explicit joins, relationships between tables are explicitly stated using `JOIN` and `ON` keywords, providing a structured approach to querying data. On the other hand, implicit joins, while simpler in syntax, may sacrifice clarity and may be less flexible, potentially leading to performance issues and difficulties in troubleshooting complex queries. Therefore, while implicit joins may be suitable for simpler queries, explicit joins are generally preferred for more complex scenarios where precision and control over join conditions are essential.