Explicit vs. Implicit Joins in SQL: What’s the Difference?

When working with SQL, understanding joins is crucial for retrieving related data from multiple tables. Two common ways to write joins are explicit joins and implicit joins. In this article, we’ll explore the differences between them, when to use each, and best practices for writing efficient SQL queries.

What Are SQL Joins?

SQL joins are used to combine records from two or more tables based on a related column. The most common types of joins are:

INNER JOIN: Returns only matching records between tables.
LEFT OUTER JOIN: Returns all records from the left table and matching records from the right.
RIGHT OUTER JOIN: Returns all records from the right table and matching records from the left.
FULL OUTER JOIN: Returns all records from both tables, with NULL where there is no match.
CROSS JOIN: Returns every possible combination of rows from both tables.

What Is an Explicit Join?

An explicit join uses the JOIN keyword along with an ON condition to specify how tables should be joined.

Example: Explicit INNER JOIN

SELECT e.entityid, e.companyname, t.tranid, t.total  
FROM entity e  
INNER JOIN transaction t ON e.id = t.entity;

Clear structure
Better readability
Explicitly defines the join condition

Explicit LEFT OUTER JOIN Example

SELECT e.entityid, e.companyname, t.tranid, t.total  
FROM entity e  
LEFT OUTER JOIN transaction t ON e.id = t.entity;

Returns all customers and their transactions (if any).
If a customer has no transactions, NULL appears in the transaction columns.

What Is an Implicit Join?

An implicit join uses the FROM clause to list multiple tables and applies the join condition in the WHERE clause.

Example: Implicit INNER JOIN

SELECT e.entityid, e.companyname, t.tranid, t.total  
FROM entity e, transaction t  
WHERE e.id = t.entity;

Issues with Implicit Joins:

Harder to read and maintain.
Can lead to accidental CROSS JOINs if the WHERE condition is missing.

Explicit vs. Implicit Joins: Which One to Use?

Best Practice: Always Use Explicit Joins!

Modern SQL databases optimize explicit joins better.
It’s easier to maintain and debug queries.
It prevents unintended CROSS JOINs when the WHERE clause is missing.