Subquery ซ้อนใน Subquery เรียกว่า Nested Subquery ตัวอย่างใช้ตาราง Cars, Customers, Reservations:
mysql> Select * from Cars; +------+--------------+---------+ | ID | Name | Price | +------+--------------+---------+ | 1 | Nexa | 750000 | | 2 | Maruti Swift | 450000 | | 3 | BMW | 4450000 | | 4 | VOLVO | 2250000 | | 5 | Alto | 250000 | | 6 | Skoda | 1250000 | | 7 | Toyota | 2400000 | | 8 | Ford | 1100000 | +------+--------------+---------+ 8 rows in set (0.02 sec) mysql> Select * from Customers; +-------------+----------+ | Customer_Id | Name | +-------------+----------+ | 1 | Rahul | | 2 | Yashpal | | 3 | Gaurav | | 4 | Virender | +-------------+----------+ 4 rows in set (0.00 sec) mysql> Select * from Reservations; +------+-------------+------------+ | ID | Customer_id | Day | +------+-------------+------------+ | 1 | 1 | 2017-12-30 | | 2 | 2 | 2017-12-28 | | 3 | 2 | 2017-12-29 | | 4 | 1 | 2017-12-25 | | 5 | 3 | 2017-12-26 | +------+-------------+------------+ 5 rows in set (0.00 sec)
Nested Subquery
mysql> Select * from Customers where customer_id IN (Select Customer_id from reservations where id = (Select id from cars WHERE name = 'BMW')); +-------------+---------+ | Customer_Id | Name | +-------------+---------+ | 2 | Yashpal | +-------------+---------+ 1 row in set (0.00 sec)
ลำดับการทำงาน: Inner-most: หา ID ของ BMW (ได้ 3) -> Middle: หา Customer_id จาก Reservations ที่ id=3 (ได้ 2) -> Outer: หา Customer ที่ customer_id=2 (ได้ Yashpal)
