When you define a Primary Key on a table:

Uniqueness: ✅
- No two rows can have the same value (or same combination, for composite keys).
NOT NULL: ✅
- No NULLs allowed in any column that is part of the Primary Key.
- Every row must have a real, non-missing value for the Primary Key.
Automatic Index: ✅
- A unique index is automatically created behind the scenes.
- This makes finding, updating, and deleting rows much faster.
Data Integrity: ✅
- Ensures your data has a reliable way to uniquely identify each row.
Foreign Key Relationships: ✅
- Other tables can create foreign keys pointing to this Primary Key.
- This is how you build relationships between tables (like Users → Orders).

📝 In Short

Feature	Primary Key Ensures
Uniqueness	No duplicate key values.
Not Null	No NULL allowed in the key.
Fast Lookup	Creates a unique index for fast access.
Data Integrity	Each row is reliably identified.
Relationships	Other tables can refer to it (foreign keys).

Example

CREATE TABLE Users (
    user_id INT PRIMARY KEY,
    name VARCHAR(100)
);

✅ What happens now: