How to use GREATEST function with Over Partition by in Oracle - oracle

Question

In the below code I want to select customer_name, location, gender and address along with customerid, aread_code.
select
customerid, aread_code, GREATEST(MAX(productid), MAX(itemid))
from
CUSTOMER C
inner join
ORDER O ON c.custid = o.custid
where
c.custtype = 'EXECUTIVE'
group
customerid, by aread_code;
I tried GREATEST function along with OVER PARTITION BY to display required columns. It's throwing an error.
Could you please help me to select the required columns.
Thank you.

Answer 1

DISCLAIMER:
When working with more than one table, qualify the columns with their table name. You haven't done so, so we don't know what of the two tables the aread_code resides in. In my answer here I assume it is the customer's area. If it isn't then you need a different answer.
ANSWER:
You group by customer_id and area code. This gives you one row per customer. And you want the maximum product/item ID from the orders table. (I suppose they are drawn from the same sequence, so you can use this ID somehow to go on from there.)
The easiest approach for this is to get the maximum ID in a subquery. Either directly in the select clause or in the from clause.
Here is how to do this in the SELECT clause:
select
c.*,
(
select greatest(max(productid), max(itemid))
from orders o
where o.custid = c.custid
) as max_id
from customer c
where c.custtype = 'EXECUTIVE';
Here is one way to do this in the FROM clause:
select
c.*,
agg.max_id
from customer c
outer apply
(
select greatest(max(productid), max(itemid)) as max_id
from orders o
where o.custid = c.custid
) agg
where c.custtype = 'EXECUTIVE';
And here is another way to do this in the FROM clause:
select
c.*,
agg.max_id
from customer c
left outer join
(
select
custid,
greatest(max(productid), max(itemid)) as max_id
from orders
group by custid
) agg on agg.custid = c.custid
where c.custtype = 'EXECUTIVE';
If you only want customers with at least one order, then I recommend the approach with the FROM clause. You'd have to turn the OUTER APPLY into a CROSS APPLY resp. the LEFT OUTER JOIN into an INNER JOIN for this.

Answer 2

There are several mistakes in your code. The main confusion is not using table alias prefix for columns. There is a group by mistake and a problem with your table name ORDER - if it is a name of a table. ORDER is a reserved word in Oracle and if it is the name of the table then you should use something like "YOUR_OWNER_NAME"."ORDER".... Here is the corected code with some sample data and result:
WITH
customers (CUSTID, PRODUCTID, AREAD_CODE, CUSTOMER_NAME, LOCATION, GENDER, ADDRESS, CUSTTYPE) AS
(
Select 1, 1, 63, 'Name 1', 'Location 1', 'M', 'Address 1', 'EXECUTIVE' From Dual Union All
Select 2, 1, 63, 'Name 1', 'Location 1', 'M', 'Address 1', 'EXECUTIVE' From Dual Union All
Select 3, 3, 63, 'Name 1', 'Location 1', 'M', 'Address 1', 'EXECUTIVE' From Dual Union All
Select 4, 7, 63, 'Name 1', 'Location 1', 'M', 'Address 1', 'EXECUTIVE' From Dual
),
orders (ORDER_ID, CUSTID, ITEMID, SOME_COLUMN) AS
(
Select 1, 1, 1, 'Some other data' From Dual Union All
Select 2, 2, 1, 'Some other data' From Dual Union All
Select 3, 3, 1, 'Some other data' From Dual Union All
Select 4, 3, 3, 'Some other data' From Dual Union All
Select 5, 4, 1, 'Some other data' From Dual Union All
Select 6, 4, 8, 'Some other data' From Dual
)
select
c.custid, c.aread_code, GREATEST(MAX(c.productid), MAX(o.itemid)) "MAX_ID"
from
CUSTOMERS C
inner join
ORDERS O ON c.custid = o.custid
where
c.custtype = 'EXECUTIVE'
group by
c.custid, c.aread_code
CUSTID AREAD_CODE MAX_ID
---------- ---------- ----------
1 63 1
4 63 8
3 63 3
2 63 1
There are different options to get the rest of the columns depending on your actual data you could use some or all of them.
Option 1 - select and group by as suggested in Beefstu's comment below
select Distinct
c.custid, c.customer_name, c.location, c.address, c.gender, c. custtype, c.aread_code,
GREATEST(MAX(c.productid), MAX(o.itemid)) "MAX_ID"
from
CUSTOMERS C
inner join
ORDERS O ON c.custid = o.custid
where
c.custtype = 'EXECUTIVE'
group by
c.custid, c.customer_name, c.location, c.address, c.gender, c. custtype, c.aread_code
order by c.custid
CUSTID CUSTOMER_NAME LOCATION ADDRESS GENDER CUSTTYPE AREAD_CODE MAX_ID
---------- ------------- ---------- --------- ------ --------- ---------- ----------
1 Name 1 Location 1 Address 1 M EXECUTIVE 63 1
2 Name 1 Location 1 Address 1 M EXECUTIVE 63 1
3 Name 1 Location 1 Address 1 M EXECUTIVE 63 3
4 Name 1 Location 1 Address 1 M EXECUTIVE 63 8
Option 2. - using analytic functions MAX() OVER() with Distinct keyword (could be performance costly with big datasets) - result is the same as above
select Distinct
c.custid, c.customer_name, c.location, c.address, c.gender, c. custtype, c.aread_code,
GREATEST(MAX(c.productid) OVER(Partition By c.custid), MAX(o.itemid) OVER(Partition By c.custid)) "MAX_ID"
from
CUSTOMERS C
inner join
ORDERS O ON c.custid = o.custid
where
c.custtype = 'EXECUTIVE'
order by c.custid
Option 3 - using left join to a subquery - see the solution offered by Thorsten Kettner