Calculating average price of items purchased by customers - sql

Question

I have three tables: customer, order and line items. They are set up as follows:
CREATE TABLE cust_account(
cust_id DECIMAL(10) NOT NULL,
first VARCHAR(30),
last VARCHAR(30),
address VARCHAR(50),
PRIMARY KEY (cust_id));
CREATE TABLE orders(
order_num DECIMAL(10) NOT NULL,
cust_id DECIMAL(10) NOT NULL,
order_date DATE,
PRIMARY KEY (order_num));
CREATE TABLE lines(
order_num DECIMAL(10) NOT NULL,
line_id DECIMAL(10) NOT NULL,
item_num DECIMAL(10) NOT NULL,
price DECIMAL(10),
PRIMARY KEY (order_id, line_id),
FOREIGN KEY (item_id) REFERENCES products);
Using Oracle, I need to write a query that presents the average item price for for those customers that made more than 5 or more purchases. This is what I've been working with:
SELECT DISTINCT cust_account.cust_id,cust_account.first, cust_account.last, lines.AVG(price) AS average_price
FROM cust_account
JOIN orders
ON cust_account.cust_id = orders.cust_id
JOIN lines
ON lines.order_num = orders.order_num
WHERE lines.item_num IN (SELECT lines.item_num
FROM lines
JOIN orders
ON lines.order_num = orders.order_num
GROUP BY lines.order_num
HAVING COUNT(DISTINCT orders.cust_id) >= 5
);

Answer 1

... INNER JOIN all your tables together
... GROUP BY customer and compute the average price of each customer's lines
... use a HAVING clause to limit the results to groups having 5 or more purchases
Query:
SELECT ca.first, ca.last, avg(l.price) avg_price
FROM cust_account ca
INNER JOIN orders o ON o.cust_id = ca.cust_id
INNER JOIN lines l ON l.order_num = o.order_number
GROUP BY ca.first, ca.last
HAVING COUNT(distinct l.line_id) >=5
-- OR, maybe your requirement is ...
-- HAVING COUNT(distinct o.order_num) >= 5
-- ... the question was a bit unclear on this point

Answer 2

I think this is it. I don't think it will work right away (I know nothing about oracle) but I think you will get the idea:
SELECT orders.cust_id,
AVG(lines.price) AS average_price
FROM lines
JOIN orders ON orders.order_num = orders.order_num
WHERE orders.cust_id IN (SELECT orders.cust_id
FROM orders
GROUP BY orders.cust_id
HAVING COUNT(*) >= 5)
GROUP BY orders.cust_id;
Subquery selects customers that have more than 5 orders.
And main query just gets all lines from all orders made by this customers.
I guess you can eliminate subquery by using HAVING DISTINCT .... Anyways, one with subquery should work just fine.
UPD.
something like this
SELECT orders.cust_id,
AVG(lines.price) AS average_price
JOIN orders ON orders.order_num = orders.order_num
GROUP BY orders.cust_id
HAVING COUNT(DISTINCT orders.id) >= 5;