Correlated Subquery Issue

Question

the following sample is from ORACLE DATABASE 10g SQL
i don't understand how it works, the bellow is the table and the query
SELECT Product_id pid, product_type_id ptid, name, price
FROM products
PID PTID NAME PRICE
1 1 Modern Science 19.95
2 1 Chemistry 30
3 2 Supernova 25.99
4 2 Tank War 13.95
5 2 Z Files 49.99
6 2 2412: The Return 14.95
7 3 Space Force 9 13.49
8 3 From Another Planet 12.99
9 4 Classical Music 10.99
10 4 Pop 3 15.99
11 4 Creative Yell 14.99
PID PTID NAME PRICE
12 My Front Line 13.49
the query
SELECT product_id pid, product_type_id ptid, name, price
FROM products outer
WHERE price >
(SELECT AVG(price)
FROM products inner
WHERE inner.product_type_id = outer.product_type_id)
PID PTID NAME PRICE
2 1 Chemistry 30
5 2 Z Files 49.99
7 3 Space Force 9 13.49
10 4 Pop 3 15.99
11 4 Creative Yell 14.99
what i don't understand is HOW and WHEN the AVG function works and HOW AVG() function knows to calculate the average by grouping the PTID not the average of ALL PRICE column.
tks for helping in advance

Answer 1

The subquery
(SELECT AVG(price)
   FROM products inner
  WHERE inner.product_type_id = outer.product_type_id)knows to create an average of all the products with the same product type because of the WHERE clause, which limits the rows returned to those with the product_type_id in the outer.
Conceptually, what happens is
- the outer query gets the first row of the table (PID=1)
- it grabs the price (19.95)
- now, it runs the subquery to determine whether this is above average
- the subquery sees that outer.product_type_id is 1, so the WHERE clause eliminates all but the first two rows (PID 1&2). The average of 19.95 and 30 is 24.975.
- Since the price of the first row is not greater than 24.975, Oracle knows that it does not meet the conditions of the WHERE clause in the outer query.
- Repeat for each row in the outer query
Obviously, since Oracle is doing set-based processing, it may not work exactly this way internally. Conceptually, though, this is how correlated subqueries work.
Justin
Distributed Database Consulting, Inc.
http://www.ddbcinc.com/askDDBC