Cross joins are used to return every combination of rows from two and more than two tables. Cross Joins are sometimes called a Cartesian product. This presentation illustrates cross join examples and applications in real life.

1. CROSS JOIN APPLICATIONS
Abdul Rahman Sherzad
Computer Science Lecturer
Herat University, Afghanistan

2. Definition
■ Cross joins are used to return
every combination of rows from
two and more than two tables.
■ Cross Joins are sometimes
called a Cartesian product.
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3. Example 1: Product
and Size
■ Suppose you want every
combination of product table
and size table, as illustrated
below:
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4. Example 2: Product
and Color
■ Suppose you want every
combination of product/item
table and color table, as
illustrated below:
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5. Example 3:
Product, Size and
Color
■ Suppose you want every
combination of product table,
size table, and color table, as
illustrated below:
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6. Example 4: Student and Subject
■ Suppose you want to print every combination of Student table and
Subject table to record their marks, as illustrated below:
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7. Example 5: Scenario
■ Calculating students’ attendance in comparison to the subjects’
requirement:
– Let’s consider an example where the faculty would like to know students’
attendance against the subjects’ requirements. Also, let’s suppose some
subjects are offered by several faculties, even different universities. Hence
students can attend those subjects at different locations.
■ In such a scenario, the cross join is required to identify those students which did not
attended some of the subjects at all. Then a left outer join is required to be used as
a final step.
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8. Example 5: Inputs
Subjects Attendance
Requirement
Students Attendance
Records
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9. Example 5: Cross Join
Subjects Attendance Requirement CROSS JOIN Students Attendance
Records
The cross join successfully
identified subject “Math” where
student “20003” did not attend it
at all.
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10. Example 5: Left Outer Join
Left Outer Join of the Cross-Join output with Students Attendance
Records total as the final step.
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11. Example 6 – Deck of Cards
■ This is another interesting example of Cross Join demonstrating the true application
of Cartesian Product
– This example may not be very practical but very useful in teaching
■ Standard Deck of Cards
– The standard playing card ranks {Ace, 2, 3, 4, 5, 6, 7, 8, 9, 10, Jack, Queen,
King} form a 13-element set.
– The card suits {space ♠, heart ♥, diamond ♦, club ♣} form a four-element
set.
– The Cross Join (Cartesian product) of these sets returns a 52-element set
consisting of 52 ordered pairs, which correspond to all 52 possible playing
cards.
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12. Example 6 – Inputs
Card suits Playing card ranks
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13. EXAMPLE 6
–
OUTPUT
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14. 14
If you do not challenge yourself,
you will never realize what you
can become.