Updates to the data: 1) add a tuple 2) delete a tuple 3) modify the values of some attributes in a tuple Updates to the data happen very frequently. Updates to the schema: relatively rare. Rather painful. Why ?
14 Schemas and Instances • Analogy with programming languages: – Schema = type/class – Instance = value/instance • Important distinction: – Database Schema = stable over long periods of time – Database Instance = changes constantly, as data is inserted/updated/deleted
15 How should we talk about relations (that is, represent them)?