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August 22, 2005: On max and group by
Your initial position
Suppose, you have a table with customers:
create table customers ( id number primary key, name varchar2(20) ); insert into customers values (1, 'Micky Mouse'); insert into customers values (2, 'Donald Duck'); insert into customers values (3, 'Dagobert');
And you have a tables with the customer's orders:
create table orders ( cust_id references customers, article_nr varchar2(20), price number(5,2) ); insert into orders values (1, '204.290', 199.00); insert into orders values (2, '923.150', 7.98); insert into orders values (3, '523.672', 299.95); insert into orders values (3, '817.543', 29.99); insert into orders values (1, '195.866', 39.90); insert into orders values (2, '674.687', 52.39); insert into orders values (1, '375.052', 17.20); insert into orders values (3, '440.291', 179.95);
Your task is now to find each customer's order with the highest price along with its respective article number and the customer's name.
The traditional approach
A traditional approach to your task is to have a subquery that groups the orders by the customers and join this subquery to the customers table:
select c.name, o.article_nr, o.price from orders o, customers c, ( select max(price) max_price, cust_id from orders group by cust_id ) subquery where o.price = subquery.max_price and c.id = subquery.cust_id;
The output of this query:
NAME ARTICLE_NR PRICE -------------------- -------------------- ---------- Micky Mouse 204.290 199 Dagobert 523.672 299.95 Donald Duck 674.687 52.39
The row_number analytic function approach
With the row_number analyic functions, the need to join the subquery (used to find the record with the maximum price) is eliminated. However, a subquery is still needed and the row_number column must be compared with 1 (one):
select c.name, s.article_nr, s.price from customers c, (select cust_id, price, article_nr, row_number() over (partition by cust_id order by price desc) r from orders ) s where s.r = 1 and s.cust_id = c.id;
This query's output is the same as the traditional query's output, although the order of the records might change.
The min() keep (dense_rank...) approach
Although the row_number approach is (imho) more readable than the traditional approach, the need for the subquery is still disturbing.
Fortunately, in the quest for the most undecipherable syntax, some ill mind came up with a construct that does away with the subquery altogether:
select c.name, max(article_nr) keep (dense_rank last order by price) article_nr, max(price) price from orders o, customers c where o.cust_id = c.id group by c.name, cust_id;
select c.name, sum(article_nr) keep (dense_rank last order by price) article_nr, max(price) price from orders o, customers c where o.cust_id = c.id group by c.name, cust_id;
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