A quick introduction to various Join algorithms in databases - Inner Join, Outer Join, Left Outer Join , Right Outer Join

1. Joins in DBMS
Join is a binary operation and a specialization of cross product between two tables. Join is an
operation that combines cross product and selection in one single statement. There are a number
of different joins in DBMS:
1. Theta Join
2. Equi Join
3. Natural Join
4. Outer Joins
Theta Join
Theta joins merges the two table on the basis of the condition represented by theta (​θ) and is
denoted by ​R1 ​⋈​θ R2​.First it does the cross product and then select the rows on the basis of given
condition. Theta joins work for all comparison operators.
EquiJoin
In equijoin, tables are merged on the basis of common attribute between them. ​When equality
comparison operator (=) is used in Theta join, then it becomes Equi-Join.
Natural Join
No comparison operator is used in Natural join. This join is much different then Cross product. Unlike
cross product, it does not keep all the attributes of both relations. Actually, natural join can be applied
only if both relations have at least one common attribute with same name and domain. Natural join
merge the tables on the basis of that common attributes where value for that attribute is same in
both relation. Whereas cross product ,theta join and equijoin all has all attributes of merging tables
and therefore having redundancy also. But natural joins keeps only one column for common attribute
in the resulting relation.

2. Employee
Name EmpId DeptName
Anil 3415 Finance
Rahul 2241 Sales
George 3401 Finance
Aman 2202 Sales
bob 1123 Executive
Department
DeptName
Ma
nag
er
Sales
Am
an
Production
Ans
h
Employee ⋈Department
Name EmpId DeptName Manager
Rahul 2241 Sales Aman
Aman 2202 Sales Aman
Outer Joins

3. All the above given joins are also known as inner-joins. Inner join keeps only the tuples satisfying the
given condition or matching attribute but in outer join, all the tuples of a relation are present in the
resulting relation based on the type of join.
Left outer join ​(R ⟕ S)
In this join, resulting relation keeps all the tuples of the left relation i.e R and For the tuples of R
having no matching tuple in S, the attributes of S are made NULL in the resulting relation.
Right outer join ( R ⟖ S)
In this join, resulting relation keeps all the tuples of the Right relation i.e S and For the tuples of S
having no matching tuple in R, the attributes of R are made NULL in the resulting relation.
Full outer join (R ⟗ S)
Full outer join actually combines the result of both left outer join and right outer join. Resulting
relation has all tuples of both relations and for non-matching tuples in both relations, their respective
attributes of other relation are made NULL​.
Employee
Name
EmpI
d
DeptName
Anil 3415 Finance
Rahul 2241 Sales
Georg
e
3401 Finance
Aman 2202 Sales
bob 1123 Executive
Department
DeptN
ame
Manag
er
Sales Aman

4. Produc
tion
Ansh
Employee ​⟗​ ​Department
Name
EmpI
d
DeptName
Manage
r
Anil 3415 Finance NULL
Rahul 2241 Sales Aman
Georg
e
3401 Finance NULL
Aman 2202 Sales Aman
bob 1123 Executive NULL
NULL NULL Production Ansh