Thursday, December 2, 2010

A Case When Table Design is Easy and Predictable

Good table design is a great foundation for a successful application stack. Table design patterns basically resolve into master tables and transaction tables. When we know a thing or two about the master tables (or entities if you prefer), we can infer a great deal about the transactions.

This blog has two tables of contents, the Topical Table of Contents and the list of Database Skills.

A Time Billing System

Imagine we have been asked to recode the company's time-billing system. Because this is for the company we work for, we have some inside knowledge about how things work. We know that:

  • There are, of course, customers.
  • ....and employees who record time
  • Each hour we record goes against a Work Order
  • There are different kinds of jobs, like project management, programming, programming management, and others.

Knowing only this, is it possible to anticipate what the system will look like? A safe answer is "no", on the claim that we will undoubtedly learn more, but this safe answer happens to be wrong. We can in fact anticipate the overall shape of the system, and new information will shift details, but it will not change the shape.

We can anticipate the nature of the transactions if we determine the upper bound of complexity and the combinatorial completeness of the system.

The Upper Bound of Complexity

We can safely assume that the big number to get right is going to be the billing rate. Our employer assumes we will get everything else right, but the billing rate is going to have them chewing their fingernails until they know we understand it and have coded it correctly.

The cool thing is that we already have enough information to establish an upper bound on the complexity of the system by looking at the master tables, where a master table is generally one that lists details about real things like people, places, things, or activities. So far we know (or think we know) about three master tables:

  • Customers
  • Employees
  • Services

Now we define the upper bound of complexity as:

The upper bound of complexity occurs when the billing rate is determined by all three master entities.

In plain English, calculating a billing rate can be as complicated as looking up a rate specific to a customer for a service for an employee but cannot be more complex than that because there are no other entities with which to work.

Combinatorially Complete

We can also anticipate all possible calculations for the billing rate by working through the complete set of combinations of master entities. This would look like the list below. Note that we are not trying to figure out right now which of these is likely to occur, we just want to get them listed out:

  • Each service has a default rate
  • Each customer has a negotiated rate
  • Each employee bills out at a default rate
  • The combination customer-service may have a rate
  • The combination customer-employee may have a rate
  • The combination customer-service-employee may have a rate (this is the upper bound of complexity, all three master entities determine the rate).

Unless we live in a super-simple world where only the first item in the list is present, we will end up dealing with several if not all of the combinations listed above.

Each of these combinations then becomes a table, and we know the billing rate will be determined by a resolution.

New Information

Now comes the big day and we interview with somebody we'll call "The Explainer" who is going to officially explain the billing system. Can he break what we already know? No. At most he can:

  • Make us aware of new master entities, perhaps there are "projects" and "contracts" that get their own billing arrangements.
  • Dispel our notions about some of the combinations by saying, "Oh we never give a customer a default rate, the default rates come out of the services."

Going in Cold

What about the case where we know absolutely nothing about an assignment when we go in to begin the interviews? We can do a good job of thinking on our feet if we draw "The Explainer" towards the master entities. As we gain confidence that we know what the master entities are, we can ask questions to probe Combinatorial Completeness and the Upper Bound of Complexity.

One caveat: This method works for transactions between master entities. When "The Explainer" starts describing something that cannot be recognized as an interaction between master entities, do not try to stuff the problem into this box, it may not fit.

What About the Application?

At this point, we can also anticipate a lot of what the application will look like. We will need maintenance screens for all of the master entities, and a really slick UI will allow for very easy editing of those various cross-reference combination tables. As long as that much is done, we are almost finished, but not yet.

There will be some billing process that pulls the time entries, finds the correct billing rate for each one, and permanently records the invoices. If we use a resolution this task is child's play to code, debug, and maintain.

Then of course there is the presentation, the actual bill. Depending on the company, these may be delivered as hardcopy or in email. That will of course have to be coded up.


There are two conclusions. First, as originally stated, many transactions can be anticipated when you know what the master entities are.

But secondly, and every bit as important, once the table design is sound, the application pretty much writes itself. On a personal note, this is probably why I do not find application coding as exciting as I once did. Once I realized that the real challenge and satisfaction was in working out the tables, the coding of the app became a bit of a drudge, it requires no judgment as far as business rules are concerned.


Kevin Stephens said...

FYI, the second resolution link on the page takes you back to the top of the page.

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