Denormalization in SQL Server for Fun and Profit

This is just a downright ugly approach and one that leads to some unpleasant UNION queries to splice together both halves of the query chain.   Another method I’ve often seen used is the “dynamic FK” approach:

Table: Students

Student (PK)

ParentKey (FK)

ParentKeyType

Column ParentKey can either be a Teacher Key or a Classroom Key; the column ParentKeyType contains a code or other value to indicate which type ParentKey really is.   This is an even uglier approach that will eventually drive you to a dependence on illegal street drugs and an early grave.  Let’s try for something better.

The key here is to more closely model the real situation.  If the school may split classes into subclasses, they’re effectively creating a new “virtual” sub-classroom.  So let’s create a table for that entity, and attach all our other tables to it:

Table: Classrooms

Classroom  (PK)

Teacher  (FK)

Table: Subclasses

Subclass  (PK)

Classroom  (FK)

Table: Teachers

Teacher  (PK)

Subclass  (FK)

Table: Students

Student  (PK)

Subclass  (FK)

How well does this schema handle the current business rules, as well as each of the two possible future variants? 

Current Rule:  Each teacher assigned to single classroom; each student assigned to single teacher.   In this case we create a UNIQUE constraint enforcing one subclass per class, and a second UNIQUE constraint enforcing one teacher per subclass.  To make the model a little easier to use, we might also want a trigger on Classrooms to automatically create and manage the matching subclass row for each class.    Even though students aren’t directly assigned to teachers or classes, the constraints create a 1:1 relationship that allow us to enforce that relationship

Variant #1: Each student assigned to a single classroom; one or more teachers per class.  In this case, we drop the UNIQUE constraint on teachers, allowing multiple teachers to be assigned to each subclass.

Variant #2: Each student assigned to a single teacher; one or more teachers per class.   Here, we leave the UNIQUE constraint on teachers, but drop the constraint on subclasses, and the classrooms trigger that synchronizes subclasses.    When multiple teachers are assigned to a class, we insert a new subclass row and assign the second teacher to it.  This allows us to enforce the one-teacher-per-student rule, but still allow teachers to share classrooms.

So there you have it: a data model that flexibly handles three entirely different sets of business logic, without being fully normalized for any of them.    You can now rapidly address future requirement changes without having to do more than drop a constraint.

Conclusion

I hope this article has shown you that thoughtful, targeted denormalization can often be a significant benefit to future-proofing your applications.    Developers often plan ahead  by increasing column lengths or including additional ones, but  the changes these prevent are minor compared to the significant restructuring that you can prevent through denormalization.  So plan ahead, and think for the future.

However, the more common reason to denormalize is to increase application performance, not future-proof.    In the second half of this article, I’ll show the magnitude of performance gains you can achieve from denormalization.  They’re larger than you might think.

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