PL SQL function that includes multiple tables - oracle

Question

I'm new to PL SQL and have to write a function, which has customer_id as an input and has to output a product_name of the best selling product for that customer_id.
The schema looks like this:
I found a lot of simple examples where it includes two tables, but I can't seem to find one where you have to do multiple joins and use a function, while selecting only the best selling product.
I could paste a lot of very bad code here and how I tried to approach this, but this seems to be a bit over my head for current knowledge, since I've been learning PL SQL for less than 3 days now and got this task.

Answer 1

With some sample data (minimal column set):
SQL> select * from products order by product_id;
PRODUCT_ID PRODUCT_NAME
---------- ----------------
1 BMW
2 Audi
SQL> select * From order_items;
PRODUCT_ID CUSTOM QUANTITY UNIT_PRICE
---------- ------ ---------- ----------
1 Little 100 1
1 Little 200 2
2 Foot 300 3
If we check some totals:
SQL> select o.product_id,
2 o.customer_id,
3 sum(o.quantity * o.unit_price) total
4 from order_items o
5 group by o.product_id, o.customer_id;
PRODUCT_ID CUSTOM TOTAL
---------- ------ ----------
2 Little 400
1 Little 100
2 Foot 900
SQL>
It says that
for customer Little, product 2 was sold with total = 400 - that's our choice for Little
for customer Little, product 1 was sold with total = 100
for customer Foot, product 2 was sold with total = 900 - that's our choice for Foot
Query might then look like this:
temp CTE calculates totals per each customer
rank_them CTE ranks them in descending order per each customer; row_number so that you get only one product, even if there are ties
finally, select the one that ranks as the highest
SQL> with
2 temp as
3 (select o.product_id,
4 o.customer_id,
5 sum(o.quantity * o.unit_price) total
6 from order_items o
7 group by o.product_id, o.customer_id
8 ),
9 rank_them as
10 (select t.customer_id,
11 t.product_id,
12 row_number() over (partition by t.customer_id order by t.total desc) rn
13 from temp t
14 )
15 select * From rank_them;
CUSTOM PRODUCT_ID RN
------ ---------- ----------
Foot 2 1 --> for Foot, product 2 ranks as the highest
Little 2 1 --> for Little, product 1 ranks as the highest
Little 1 2
SQL>
Moved to a function:
SQL> create or replace function f_product (par_customer_id in order_items.customer_id%type)
2 return products.product_name%type
3 is
4 retval products.product_name%type;
5 begin
6 with
7 temp as
8 (select o.product_id,
9 o.customer_id,
10 sum(o.quantity * o.unit_price) total
11 from order_items o
12 group by o.product_id, o.customer_id
13 ),
14 rank_them as
15 (select t.customer_id,
16 t.product_id,
17 row_number() over (partition by t.customer_id order by t.total desc) rn
18 from temp t
19 )
20 select p.product_name
21 into retval
22 from rank_them r join products p on p.product_id = r.product_id
23 where r.customer_id = par_customer_id
24 and r.rn = 1;
25
26 return retval;
27 end;
28 /
Function created.
SQL>
Testing:
SQL> select f_product ('Little') result from dual;
RESULT
--------------------------------------------------------------------------------
Audi
SQL> select f_product ('Foot') result from dual;
RESULT
--------------------------------------------------------------------------------
Audi
SQL>
Now, you can improve it so that you'd care about no data found issue (when customer didn't buy anything), ties (but you'd then return a collection or a refcursor instead of a scalar value) etc.
[EDIT] I'm sorry, ORDERS table has to be included into the temp CTE; your data model is correct, you don't have to do anything about it - my query was wrong (small screen + late hours issue; not a real excuse, just saying).
So:
with
temp as
(select i.product_id,
o.customer_id,
sum(i.quantity * i.unit_price) total
from order_items i join orders o on o.order_id = i.order_id
group by i.product_id, o.customer_id
),
The rest of my code is - otherwise - unmodified.