Database Concepts

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Q1: What is a Database?

A1: A Database is computerized abstracted data stored in a persistent repository.

Q2: What does abstracted data mean?

A2: It's the way we perceive data to be organized in the repository.

Q3: What does persistent repository mean.

A3: Persistent repository means permanent storage, usually on the hard drive.

Q4: What is the most popular data abstraction used today?

A4: The dominant one is called Relational Database.

Q5: How is data abstracted in a Relational Database?

A5: In a Relational Database data is abstracted into tables of records.

Q6: Can you give me an example?

A6: Sure, click Suppliers-Parts-Jobs-Shipments database.

Q7. So those tables don't really exist, do they?

A7. That's right, each time we open a database, the tables are generated by the software. On the hard drive the data is stored in indexed files or hash tables or something else.

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Q8. What's so great about abstracting data into tables of records?

A8: When updating and retrieving data we only have to know the names of the table(s) and column(s) and some restrictions which row(s) we want. We do not need to know where the data is or how to get to it or how it's stored. To update or retrieve data, a language called SQL (Structure Query Language) is used. So, if the data is moved or reformatted then neither the user nor an application program that uses the data has to change anything. This is called Data Independence, i.e. applications and we are independent (free of) the details on how and where data is stored.

Q9. How is the abstraction achieved in practice?

A9. Abstraction is achieved with a software layer above the physical level. This software layer is part of a Database Management System (DBMS) sold by database vendors. There is a third layer, the presentation layer. The presentation layer is written with a Graphical User Interface tool such as Visual Basic. Here is the diagram showing the three levels of a database management system:

Three-Level architecture.

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