Physical Data Dependence

Physical data independence
The logical definition of the data remains unchanged, even when we make changes to the actual implementation

Imagine a database like a filing cabinet:
- You (the user) just ask, “Hey, give me Alice’s file.”
- You don’t care where in the cabinet the file is kept, how it’s arranged, or how it’s labeled inside — you just want the info.

📦 SQL Example:

You have this table:

Customers (ID, Name, Email)

And you run:

SELECT Name FROM Customers WHERE ID = 5;

Now imagine the database admin: - Moves that table to a faster disk - Adds an index on ID to make the query faster - Stores some rows in a different file for performance

🔐 Query still works the same:

You don’t have to change it
You still get the same result
You don’t even know anything changed inside