DBMS - Joins


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We understand the benefits of taking a Cartesian product of two relations, which gives us all the possible tuples that are paired together. But it might not be feasible for us in certain cases to take a Cartesian product where we encounter huge relations with thousands of tuples having a considerable large number of attributes.

Join is a combination of a Cartesian product followed by a selection process. A Join operation pairs two tuples from different relations, if and only if a given join condition is satisfied.

We will briefly describe various join types in the following sections.

Theta (θ) Join

Theta join combines tuples from different relations provided they satisfy the theta condition. The join condition is denoted by the symbol θ.

Notation

R1 ⋈θ R2

R1 and R2 are relations having attributes (A1, A2, .., An) and (B1, B2,.. ,Bn) such that the attributes don’t have anything in common, that is R1 ∩ R2 = Φ.

Theta join can use all kinds of comparison operators.

Student
SID Name Std
101 Alex 10
102 Maria 11
Subjects
Class Subject
10 Math
10 English
11 Music
11 Sports

Student_Detail −

STUDENT Student.Std = Subject.Class SUBJECT

Student_detail
SID Name Std Class Subject
101 Alex 10 10 Math
101 Alex 10 10 English
102 Maria 11 11 Music
102 Maria 11 11 Sports

Equijoin

When Theta join uses only equality comparison operator, it is said to be equijoin. The above example corresponds to equijoin.

Natural Join ()

Natural join does not use any comparison operator. It does not concatenate the way a Cartesian product does. We can perform a Natural Join only if there is at least one common attribute that exists between two relations. In addition, the attributes must have the same name and domain.

Natural join acts on those matching attributes where the values of attributes in both the relations are same.

Courses
CID Course Dept
CS01 Database CS
ME01 Mechanics ME
EE01 Electronics EE
HoD
Dept Head
CS Alex
ME Maya
EE Mira
Courses ⋈ HoD
Dept CID Course Head
CS CS01 Database Alex
ME ME01 Mechanics Maya
EE EE01 Electronics Mira

Outer Joins

Theta Join, Equijoin, and Natural Join are called inner joins. An inner join includes only those tuples with matching attributes and the rest are discarded in the resulting relation. Therefore, we need to use outer joins to include all the tuples from the participating relations in the resulting relation. There are three kinds of outer joins − left outer join, right outer join, and full outer join.

Left Outer Join(R Left Outer Join S)

All the tuples from the Left relation, R, are included in the resulting relation. If there are tuples in R without any matching tuple in the Right relation S, then the S-attributes of the resulting relation are made NULL.

Left
A B
100 Database
101 Mechanics
102 Electronics
Right
A B
100 Alex
102 Maya
104 Mira
Courses Left Outer Join HoD
A B C D
100 Database 100 Alex
101 Mechanics --- ---
102 Electronics 102 Maya

Right Outer Join: ( R Right Outer Join S )

All the tuples from the Right relation, S, are included in the resulting relation. If there are tuples in S without any matching tuple in R, then the R-attributes of resulting relation are made NULL.

Courses Right Outer Join HoD
A B C D
100 Database 100 Alex
102 Electronics 102 Maya
--- --- 104 Mira

Full Outer Join: ( R Full Outer Join S)

All the tuples from both participating relations are included in the resulting relation. If there are no matching tuples for both relations, their respective unmatched attributes are made NULL.

Courses Full Outer Join HoD
A B C D
100 Database 100 Alex
101 Mechanics --- ---
102 Electronics 102 Maya
--- --- 104 Mira


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