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✅ Database Basics: Primary Key and Foreign Key

Why Keys Exist
Databases store millions of rows. Keys help identify and connect data correctly. Without keys, data breaks fast.

Primary Key
- A primary key uniquely identifies each row in a table
- No two rows share the same primary key
- It never stays empty

Example Table: customers
- Columns: customer_id, name, email, city
- Primary key: customer_id

Why Primary Key Matters
- Prevents duplicate records
- Helps find a row fast
- Keeps data consistent

Foreign Key
- A foreign key links one table to another
- It creates relationships between tables

Example Table: orders
- Columns: order_id, customer_id, order_date, amount
- Foreign key: customer_id

Relationship Explained
- customers.customer_id is primary key
- orders.customer_id is foreign key
- One customer has many orders

Why Foreign Keys Matter
- Enable joins
- Prevent orphan records
- Maintain data integrity

Simple Join Idea
SQL matches customer_id in both tables. This gives customer name with order amount.

Where Beginners Go Wrong
- Using names instead of IDs
- Allowing duplicate primary keys
- Ignoring missing foreign key values

Checkpoint
- You understand primary keys
- You understand foreign keys
- You know how tables connect

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