What is the three level architecture also known as
ANSI-SPARC architecture.
What are the three levels of architecture
- External (how users see the database)
- Conceptional (logical structure)
- Internal (physical storage)
What does the external view describe
the part of the data base relevant to users
What does the conceptual level describe
What data is stored and the relationships among the data
What does the internal level describe
How data is stored, include file organisation, indexes and access paths
What are the two main objectives of the thee level architecture
data abstraction
data independence
What is data abstraction
Hiding storage details and presenting users with a conceptual view of the data base
what is data independence
The capacity that upper levels are unaffected by changes to lower level.
What are the levels of abstraction in a DBMS
- Physical level (internal schema)
- Logical level (conceptual scheme)
- view level (External schema)
What is the internal schema
the lowest level of abstraction and describes how the data is physically stored
what is conceptual schema
the middle level of abstraction and describes what data is stored and the relationships between them
What is external schema
the top level of abstraction and describes what a specific user sees
Give an ATM as an example of data abstraction
Analogy: Think of an ATM.
- Physical level: The cash, circuitry, and storage hardware.
- Logical level: The bank database defines accounts, balances, and transactions.
- View level: You only see your own balance and transaction history.
Define physical data independence
Change internal schema without having to change conceptual schema.
Define logical data independence
Change internal schema without having to change conceptual schema.
give an example for physical data independence
Example:
- The DB admin moves data from hard drives to SSDs, or changes file indexing.
- Applications using tables like Customer(CustomerID, Name, Email) still work.
give an analogy for example data indepepednance
Example:
- You add a new attribute PhoneNumber to Customer.
- The HR app still works because it only queries CustomerID and Name.
- Users don’t need to rewrite their code.
What is a database schema
the description of the structure of the database
What is the data base state
The content of a DB at a moment in time.
Give an example of schema vs. state.
Schema: Employee(emp_id, emp_name, address, dept_id)
State: the actual list of employees stored at a given time.
What is a data model
a set of concepts to describe data, relationship among data and constraints
What are the three categories of data models
- Conceptual
- Logical
- Physical
What is a conceptual data model
A conceptual data model is the highest-level view of a database.
It describes the big picture: what entities exist, what attributes they have, and how they relate to one another.
Wet is a logical data model
Describes data structures (records, sets, tables, objects), independent of physical storage.
