Using nested query instead of Join method - sql

Question

I have 3 tables.
Car (PK: CarNo)
sales (FK:CarNo)
purchase (FK:CarNo)
A system like a car showroom. Cars are bought from customers and these cars are sold to customers. I want the SQL query that finds the cars whose sale price is less than the purchase price.
I created the following query using inner join, but I don't want to use join. How can I do it using nested query?
select DISTINCT * from car
left join sale on car.carNo=sale.carNo
left join purchase on sale.carNo=purchase.carNo
where sale.salePrice<purchase.purchasePrice;

Answer 1

Assuming a car can be sold or purchased only once, what you have is fine, though you shouldn't use DISTINCT and those should be inner joins.
select car.*
from car,
join sale on car.carNo = sale.CarNo
join purchase on sale.carNo = purchase.charNo
where sale.salePrice < purchase.purchasePrice
if you really wanted to use subqueries:
select car.*
from car
where (SELECT MAX(sale.salePrice)
FROM sale
WHERE sale.CarNo = car.CarNo) <
(SELECT MAX(purchase.purchasePrice)
FROM purchase
WHERE purchase.CarNo = car.CarNo)
Of course, if a car can be sold or purchased more than once, than you will need more than just CarNo, you'll need some date logic of some kind to make sure you are comparing the right sale with the right purchase.