Values that are *sometimes* calculated (1 Viewer)

[hokiewalrus]

I've been pulling my hair recently with table design with regards to values that are sometimes based on other data but sometimes not.

The problem is this: the company I'm consulting for enters in the hours each employee worked on each job. The total hours for the day is sometimes accepted as is, sometimes overridden by a manager. At the end each week, another manager looks at those numbers and sometimes accepts them and sometimes overrides either the value for a day or the total for the week.

Ethical implications and actual value aside, this is what they do and they aren't going to change.

The real issue arises when a change gets made all the way down at the hour level, which sometimes changes the total for the day, which in turn then sometimes changes the value for the week.

Short of storing a calculated total with an "override" boolean flag and tons of recalculate routines that get called when needed, I'm not sure what the best approach is here.

I've already given myself a concussion banging my head against the wall trying to get them to base their business in some semblance of reality, at this point I just have to try and give them what they want.

What is the best way to handle occasionally calculated values?

 

[georgedwilkinson]

I don't believe I truly understand your problem but if you are storing calculated values, stop...don't do that.

You should always re-calculate your data unless the state of your data is static.

 

[hokiewalrus]

Let me try and explain better:

My problem is that I have values that are *sometimes* based on underlying data, but sometimes they are not. There are additional values that are *sometimes* based on that first level of data.

So:
Level 1: Number of hours per job per day
Level 2: Total of Level 1, unless manually overridden
Level 3: Total of Level 2, unless manually overridden

Now a few days down the road, a value in Level 1 can change. That can potentially change the value of Level 2, which can in turn potentially change the value of Level 3.

I'm just not sure how to store and organize data with that much potential for change.

 

[georgedwilkinson]

Let me try and explain better:
Number of hours per job per day is a calculated value. You should not store static calculated values in your database.

 

[Atomic Shrimp]

Except the value the OP is talking about is sometimes the straight calculated value, and sometimes it's overridden by some other value that is not the pure result of the calculation.

How should that be handled?

 

[georgedwilkinson]

Store the pure data. Do calculations based off the pure data.

When I think about pure data and a time management system, I never can come to a single event that causes "Number of hours per day per job". I can perceive that a person will begin work on a job and then stop working on that job. That is the data you store: begin and end time (along with the job information/FK and person information/FK). Then you have everything you need to do calculations on the fly.

 

[Atomic Shrimp]

So in this case, store the manual override as some kind of offset, zero if no override has been imposed? - and make it part of the calculation?

 

[georgedwilkinson]

I'm not sure if I should comment on the "manual override" without knowing more about it.

There is a reason experts say not to store calculated values (except under certain conditions). The OP has run into one of those reasons.

But he hasn't given us enough information to assist him with ALL the things that must be captured in his database.

 

[Atomic Shrimp]

The manual override thing is the whole point of the thread - this isn't really a question about storing calculated values, but rather, handling exceptions to calculated values.

So I would say a workable approach here is to store an extra row - you've presumably already got a number of rows describing blocks of worked hours - store another one (where appropriate) with a block of adjustment hours (positive or negative), then just total the records to get the adjusted worked hours.

 

[georgedwilkinson]

Ok, re-reading the original post and with Shrimp's corrections, I recommend that you:
1. Store the initial incoming value in a table with an autonumber PK
2. Store the corrected value in the same table, disabling (not deleting) the initial value and pointing to the "parent" value from the "corrected" entry.
3. Do the same thing for the next corrected value.

All of the values will then be available for review by...whoever.

 

[Atomic Shrimp]

I've been pulling my hair recently with table design with regards to values that are sometimes based on other data but sometimes not

....

What is the best way to handle occasionally calculated values?

So make them always based on data.

Suppose your hours worked table looks something like this:
Date,Hours
01/12/09,8
02/12/09,7.5
03/12/09,7.25
04/12/09,8
05/12/09,8

...and the manager looks at it and decides 39 hours is too much, he thinks it should be 38, just store another row with his adjustment, so:
Date,Hours
01/12/09,8
02/12/09,7.5
03/12/09,7.25
04/12/09,8
05/12/09,8
05/12/09,-1

You might also want to add a column to describe the type of record, so:
Date,Hours
01/12/09,8, Standard Hours
02/12/09,7.5, Standard Hours
03/12/09,7.25, Standard Hours
04/12/09,8, Standard Hours
05/12/09,8, Standard Hours
05/12/09,-1, Manager Adjustment

(shown above as text for illustration, you'd just store a foreign key to a table of record types in reality)

That way, the total hours worked for the week is still a purely calculated value - and you can calculate it with or without the manager adjustment by simply applying criteria.
It also preserves the integrity of the system by accounting for everything, whether it's normal or overridden

 

[gemma-the-husky]

Personally, i think you have to store the original unadjusted data. I think you should also store this same data initially in another field as working data, and base all your calcs off the working data. If the working data is changed, I would then store details of who changed it, and when it was changed (and maybe record all subsequnt changes also)That way you have a clear audit of changes in the database.

 

[hokiewalrus]

Thanks for all the replies.

In the end I think I'm going to just keep all the raw data (without totals), and then have another table of the manual overrides, if any. This will allow me to keep all the raw data should it be needed, easily pull out the most recent override, and keep an audit trail of overrides so you can see who did what and when, hopefully all with some semblance of normalization.

Thanks for all the help, it really cleared things up a lot for me.