Physical structure — specify data types and constraints.

The physical structure of a database describes specific data types and constraints for tables and columns that ensure integrity and optimization.

Example:

• Data types: INT, VARCHAR(255), DATE, BOOLEAN, DECIMAL(10,2)
• Constraints:
  • PRIMARY KEY — a unique identifier for a record
  • FOREIGN KEY — a reference to another table to ensure connectivity
  • NOT NULL — a mandatory field
  • UNIQUE — uniqueness of the value in the column
  • CHECK — validation of the value based on a condition

For example, a users table:

Field	Data Type	Constraints
id	INT	PRIMARY KEY, NOT NULL
username	VARCHAR(50)	UNIQUE, NOT NULL
email	VARCHAR(100)	UNIQUE, NOT NULL
created_at	TIMESTAMP	NOT NULL

This description helps understand how data will be stored and what rules are applied for their correctness.