1. Uniqueness: ✅
   • No two rows can have the same value (or same combination, for composite keys).
2. 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.
3. Automatic Index: ✅
   • A unique index is automatically created behind the scenes.
   • This makes finding, updating, and deleting rows much faster.
4. Data Integrity: ✅
   • Ensures your data has a reliable way to uniquely identify each row.
5. 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).

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Example

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

✅ What happens now:

• user_id must be unique.
• user_id cannot be NULL.
• A unique index on user_id is created automatically.
• You can easily do:

user_id must be unique.

user_id cannot be NULL.

A unique index on user_id is created automatically.

You can easily do:

SELECT * FROM Users WHERE user_id = 123;

and it will be fast.

So — to your question:

When I define a primary key, it means I cannot insert a record with the same primary key, and that is it?

✅ YES — plus:

• No NULLs allowed in that column.
• Fast searching thanks to the index.
• Safe relationships via foreign keys.

It’s not only about uniqueness — it’s about uniqueness + not null + fast access + relational integrity.

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Visual Analogy

Primary Key is like a passport number:

• Unique — no two people can have the same passport number.
• Mandatory — you must have one (can’t be NULL).
• Index — the government database can quickly find you using it.
• Link — other systems (airlines, visa offices) can reference it.