Notation:
R1 ?? R2
R1 and R2 are relations with their attributes (A1, A2, .., An ) and (B1, B2,.. ,Bn) such that no attribute matches that is R1 ? R2 = ? Here ? is condition in form of set of conditions C.
Theta join can use all kinds of comparison operators.
Student
SID Name Std
101 Alex 10
102 Maria 11
[Table: Student Relation]
Subjects
Class Subject
10 Math
10 English
11 Music
11 Sports
[Table: Subjects Relation]
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
[Table: Output of theta join]
Equi-Join
When Theta join uses only equality comparison operator it is said to be Equi-Join. The above example conrresponds to equi-join
Natural Join ( ? )
Natural join does not use any comparison operator. It does not concatenate the way Cartesian product does. Instead, Natural Join can only be performed if the there is at least one common attribute exists between relation. Those attributes must have same name and domain.
Natural join acts on those matching attributes where the values of attributes in both relation is same.
Courses
CID Course Dept
CS01 Database CS
ME01 Mechanics ME
EE01 Electronics EE
[Table: Relation Courses]
HoD
Dept Head
CS Alex
ME Maya
EE Mira
[Table: Relation HoD]
Courses ? HoD
Dept CID Course Head
CS CS01 Database Alex
ME ME01 Mechanics Maya
EE EE01 Electronics Mira
[Table: Relation Courses ? HoD]
Outer Joins
All joins mentioned above, that is Theta Join, Equi Join and Natural Join are called inner-joins. An inner-join process includes only tuples with matching attributes, rest are discarded in resulting relation. There exists methods by which all tuples of any relation are included in the resulting relation.
There are three kinds of outer joins:
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
|