[rehack]

This is what I understood (s)he means

Scenario: Laptop's price is initially $800 and then increases to $850 before dropping to $825

Design 1: with columns Amount and Kind will just have a single row which will get modified with the latest value

Design 2: Will have 3 rows in the above case

Difference | Kind | Time stamp

800 | 'Laptop' | ts 1

+50 | 'Laptop' | ts 2

-25 | 'Laptop' | ts 3

So this latter design has 'undo' builtin

edit: formating

[ajuc]

Yes, that's what I've meant. This also gives you easy accountability (rows can have information like user_id, ip, timestamp).