Theta Join (Relational Algebra)

relational-algebra

Definition

Theta Join (Relational Algebra)

The theta join $R{\bowtie }_{\theta }S$ combines two relations via a joining predicate $\theta$ by combining only tuples for which $\theta$ holds.

Example

Given two relations $students$ and $enrollments$:

Students Relation:

StudentID	Name	Major
S101	Alice	Physics
S102	Bob	CS
S103	Charlie	Biology

Enrolments Relation:

StudentID	CourseID	Grade
S101	C22	A
S102	C15	B
S102	C22	A
S104	C15	C

Join on StudentID, thus:

$$\theta :=students.student\_id=enrolments.student\_id$$

The resulting relation $students{\bowtie }_{\theta }enrolments$ is:

StudentID	Name	Major	CourseID	Grade
S101	Alice	Physics	C22	A
S102	Bob	CS	C15	B
S102	Bob	CS	C22	A