Difference between join, Inner join,Equijoin and Natural Join

An inner join is a join with a join condition that may contain both equality or non-equality sign whereas an equijoin is a join with a join condition that only contain only equality sign.
So we can say an equijoin is a type of inner join containing (Equal)= operator in the join condition.

It is good to know the difference between join and INNER JOIN keywoed. Actually there is no difference. If we write JOIN then by default INNER JOIN is performed. In the example it is also shown.
The following example will make you more clear.

In this example I used data as in example of Difference between Inner join and Outer join
SQL> select s.name,d.deptname from dept d, student s where d.deptid=s.deptid;
NAME DEPTNAME
--------------- ----------
Rafi CSE
Arju CSE

This example represents both INNER join and equijoin.
SQL> select s.name,d.deptname from dept d INNER JOIN student s on d.deptid=s.deptid;
NAME DEPTNAME
--------------- ----------
Rafi CSE
Arju CSE

Above example also represents both INNER join and equijoin.
SQL> select s.name,d.deptname from dept d INNER JOIN student s on d.deptid<>s.deptid;

NAME DEPTNAME
--------------- ----------
Rafi EEE
Raju EEE
Arju EEE
Raju CSE

Above example represents an inner join but not a equijoin.
SQL> select s.name,d.deptname from dept d JOIN student s on d.deptid<>s.deptid;

NAME DEPTNAME
--------------- ----------
Rafi EEE
Raju EEE
Arju EEE
Raju CSE

Above example show JOIN and INNER join keyword is same. If we don't specify INNER then by default inner join is performed.

Now let's have a look at natural join.
SQL> desc dept;
Name Null? Type
----------------------------------------- -------- ----------------------------
DEPTID NUMBER
DEPTNAME VARCHAR2(10)

SQL> desc student;
Name Null? Type
----------------------------------------- -------- ----------------------------
STDID NUMBER
NAME VARCHAR2(15)
DEPTID NUMBER

After describing both table we see both table have a same column name deptid. Now if I perform natural join then tables with same column name is joined.
SQL> select s.name,d.deptname from dept d NATURAL JOIN student s ;
NAME DEPTNAME
--------------- ----------
Rafi CSE
Arju CSE

As both table have same column deptid so deptid is joined.

Now I rename deptid column and see the result. We will notice in that case inner join will be performed.

SQL> alter table student rename column deptid to deptid1;
Table altered.

SQL> select s.name,d.deptname from dept d NATURAL JOIN student s ;

NAME DEPTNAME
--------------- ----------
Rafi EEE
Raju EEE
Arju EEE
Rafi CSE
Raju CSE
Arju CSE

6 rows selected.

As both table don't have same column name so normal join/inner join is performed.
SQL> select s.name,d.deptname from dept d, student s ;

NAME DEPTNAME
--------------- ----------
Rafi EEE
Raju EEE
Arju EEE
Rafi CSE
Raju CSE
Arju CSE

6 rows selected.

SQL> select s.name,d.deptname from dept d CROSS JOIN student s ;

NAME DEPTNAME
--------------- ----------
Rafi EEE
Raju EEE
Arju EEE
Rafi CSE
Raju CSE
Arju CSE

6 rows selected.
Related Document:
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Difference between Inner join and Outer join
Joins in Oracle