Difference between Inner Join and Equi Join and Natural Join

Estimated study time: 13 minutes. Three closely related join types, explained side by side.

These three terms often get used interchangeably, but they describe slightly different things — one is a join type, while the other two describe the condition used inside a join.

Inner Join

A join type that returns only rows with matching values in both tables. The join condition can use any comparison operator, though equality is by far the most common.

SELECT o.OrderID, c.Name
FROM Orders o
INNER JOIN Customers c ON o.CustomerID = c.CustomerID;

Equi Join

Describes the condition, not a distinct join type — any join where the condition uses the equality operator (=) is called an equi join. It's usually written as an inner join, but technically applies to outer joins too.

SELECT o.OrderID, c.Name
FROM Orders o, Customers c
WHERE o.CustomerID = c.CustomerID;

Natural Join

Automatically joins two tables based on all columns with the same name in both tables — no explicit ON condition is written at all. SQL Server doesn't support the NATURAL JOIN keyword directly (unlike MySQL or Oracle), so the same result must be written manually as an equi join.

-- MySQL / Oracle syntax
SELECT * FROM Orders NATURAL JOIN Customers;

-- Equivalent in SQL Server
SELECT * FROM Orders o
JOIN Customers c ON o.CustomerID = c.CustomerID;

Summary

• Inner Join: a join type returning only matched rows
• Equi Join: any join using an equality condition
• Natural Join: an implicit join based on matching column names