1) Subquery kya hoti hai?

Subquery = ek SELECT ke andar dusra SELECT.

Socho jaise:

“Pehle andar se data nikaalo, phir bahar wali query us result pe kaam kare.”

2) “Outer query depends on inner query” — detail me

Ye line uncorrelated subquery ke liye mostly true hoti hai:

Uncorrelated subquery (andar wali independent)

Inner query ek baar run hoti hai, result nikalti hai, phir outer query use use karti hai.

Example:

SELECT*
FROM customer
WHERE id IN (SELECT cust_id FROM orders);

Correlated subquery (outer se linked) — yaha outer driving hota hai

Inner query outer row ke hisaab se bar-bar run hoti hai (conceptually).

Example:

SELECT c.id, c.name
FROM customer c
WHERE EXISTS (
SELECT 1
FROM orders o
WHERE o.cust_id= c.id
);