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Set filters

Set filters compare two groups of rows collected during a relational classification.

Instead of checking rows one by one, a set filter compares two sets of related rows and decides which rows should remain in the evaluation.

They are useful when a rule depends on how two collections of rows relate to each other.

When set filters are useful

Set filters are used when the rule depends on comparing groups of rows.

Examples:

  • Do two related entities share at least one common item?
  • Are all rows in one set also present in another set?
  • Should rows that appear in another collection be removed?
  • Do two sets contain different elements?

If the condition can be checked row by row, a classification filter is usually the better choice.

Set filters are used only when the comparison must be done between sets of rows.

How set filters work

During relational classification evaluation, the system may produce several result sets at different steps in the relation path.

A set filter compares:

  • Set A – a set produced at an earlier step
  • Set B – the set produced at the current step

The filter applies a set operation to determine which rows remain.

Example:

Tables

  • Customer
  • Booking
  • Event

Relations

Customer → Booking → Event
Customer → FavoriteEvent

A set filter could compare:

Events from bookings
∩
Favorite events

This would keep only events that appear in both sets.

Supported operations

Set filters support several standard set operations.

Intersection

A ∩ B

Keeps rows that appear in both sets.

Union

A ∪ B

Combines rows from both sets.

Difference

A − B

Keeps rows in A that do not appear in B.

Reverse difference

B − A

Keeps rows in B that do not appear in A.

Symmetric difference

A Δ B

Keeps rows that appear in only one of the sets.

Example

Tables

  • Student
  • Course
  • Enrollment

Relations

Student → Enrollment → Course
Student → AllowedCourse

Set filter:

Course ∩ AllowedCourse

Result:

Student Courses enrolled Allowed courses Courses counted
Anna Math, Physics Math Math
Erik Biology Math (none)

Only the courses present in both sets are kept.

Important constraint

Both sets being compared must contain rows from the same table.

This ensures the system can compare row identities correctly.

However, the comparison is not completely free.

A set can only be compared against the first earlier step in the relation path that produces rows of the same table type.
You cannot choose any earlier set of the same type.

Example relation path result sets:

Step 1 → Course
Step 2 → Teacher
Step 3 → Course
Step 4 → Course

At Step 4, the comparison can only use the Course set from Step 3, because it is the closest earlier step with the same table type.

For example:

Course set compared with Course set

is valid, but

Course set compared with Student set

is not allowed.

A simple way to think about set filters

Set filters answer a question like:

How do these two groups of related rows compare?

By comparing sets instead of individual rows, relational classifications can express more complex structural rules.

Reference

Related concepts