0) JOIN kyu chahiye?

RDBMS me data normalize hota hai, means:

• Customer info customer table me
• Orders orders table me
• Order items order_items table me

Ab meaningful result nikalna ho (e.g., “Har order ka customer name?”)

Tables ko combine karna padega.

JOIN = tables ko relation (FK/PK) ke basis par combine karke output banana.

FK usually join ka base hota:

orders.cust_id → customer.id

1) JOIN ka basic format

SELECT columns
FROM table1
JOIN table2
ON join_condition;

Example:

SELECT o.order_id, c.name
FROM orders o
JOIN customer c
ON o.cust_id= c.id;

Important:

• ON me join rule hota (kaise match karna hai)
• WHERE me filter hota (kaunse rows chahiye)

2) INNER JOIN