ORACLE-FORU - Difference between inner join and equi join and natural join

One of the most common interview questions that is being asked is explain Equi Join, Natural Join and Cross Join in sql. I will break the question in different meaningful easy to understand concepts

Inner Join

This is the most used join in the SQL. this join returns only those records/rows that match/exists in both the database tables.

Inner Join Example

SELECT * FROM tblEmp JOIN tblDept
ON tblEmp.DeptID = tblDept.DeptID;

Inner Join Result

tblEmp.Name

tblEmp.DeptID

tblDept.Name

tblDept.DeptID

Ram

1

HR

1

Raju

2

IT

2

Soya

2

IT

2

Sam

3

ADMIN

3

In the join condition, you can also use other operators like <,>,<>.

Equi Join

Equi join is a special type of join in which we use only equality operator. Hence, when you make a query for join using equality operator then that join query comes under Equi join.

Equi Join Example

SELECT * FROM tblEmp JOIN tblDept
ON tblEmp.DeptID = tblDept.DeptID;
--Using Clause is not supported by SQL Server
--Oracle and MySQL Query
SELECT * FROM tblEmp INNER JOIN tblDept USING(DeptID)

Equi Join Result

tblEmp.Name

tblEmp.DeptID

tblDept.Name

tblDept.DeptID

Ram

1

HR

1

Raju

2

IT

2

Soya

2

IT

2

Sam

3

ADMIN

3

Note

1. Inner join can have equality (=) and other operators (like <,>,<>) in the join condition.
2. Equi join only have equality (=) operator in the join condition.
3. Equi join can be an Inner join, Left Outer join, Right Outer join
4. The USING clause is not supported by SQL Server and Sybase. This clause is supported by Oracle and MySQL.

Natural Join

Natural join is a type of equi join which occurs implicitly by comparing all the same names columns in both tables. The join result have only one column for each pair of equally named columns.

Natural Join Example

--Run in Oracle and MySQL 
SELECT * FROM tblEmp NATURAL JOIN tblDept

Natural Join Result

DeptID

tblEmp.Name

tblDept.Name

1

Ram

HR

2

Raju

IT

2

Soya

IT

3

Sam

ADMIN

In the above join result we have only one column "DeptID" for each pair of equally named columns.

Note

1. In Natural join, you can't see what columns from both the tables will be used in the join. In Natural join, you might not get the desired result what you are expecting.
2. Natural join clause is not supported by SQL Server, it is supported by Oracle and MySQL.