Modeling Change: Price

• ·         Keeping an on-going record of transactions. – Journaling and/or Logging
• ·         A rise in value or price, especially over time - Appreciation
• ·         A decrease or loss in value, because of age, wear, or market conditions. - Depreciation

1. Comic-book collectors need to know the price history of different types of comics. This helps them decide what to purchase/collect and how much to sell their collection for.

Create an ERD (two or three entities) to model this.

There are high chances that Enddate is known here, since price fluctuation is not very drastic on time scale and we are talking of new buys rather than old stuff buy where price vary of items with same condition.

2. When a student’s grade is changed, we need to record information on the teacher who changed the grade and the reason for the change.

Start with the ENROLLMENT entity, which is the resolution of the M:M between STUDENT and CLASS.

Enrollment is there, but quite possible not graded yet. If a grade is changed, Grade in GRADELOG with most recent DateTimeGiven is honored for transcripts. Reason for change is saved in comments. It may be used to say very good in first attempt of grading also.

3. When vintage-car aficionados shop for additions to their collections, it helps to know the price history of the cars they are interested in. It may also make them consider selling one of their cars if it has greatly increased in value since they bought it.

Create an ERD (at least two entities) to model this.

We store here datetime evaluated, without end time, because even similar used condition car may be sold for two different prices. So it’s a kind of crude feed which I am storing here.