Page 4 - Grouping Records and Join in SQL

Data from two tables can be extracted by equijoin (by applying condition on cartesian product table)
or natural join.

EQUI JOIN (join in which columns are compared for equality)

Join is Cartesian Product of two tables
Equi join returns data from two tables using equality operator
1. select ename, dname
from employee, department
where employee.deptno = department.deptno
OR
select ename, dname
from employee E, department D
where E.deptno = D.deptno
2. select ename, dname
from employee E, department D
where E.deptno = D.deptno
and E.deptno = 20

E and D are table alias, temporary names of tables till the execution of SQL command.

NATURAL JOIN

Natural Join is an implicit join clause based on the common columns in the two tables being joined.
Common columns are columns that have the same name in both tables.
This is a much riskier join as with this join the query attempts to use all columns with common
names to create a join.

Consider two tables T1 and T2