如何将值与 SQL max(count) 函数匹配？

Question

I have a orderLine table looks like this

I would like to know which pizza is the best seller, and the quantity of pizza sold.

I've tried query:

select sum(quantity), pizza_name  from order_line group by pizza_name;

it returns

which is almost what I want, But when I start adding Max function, it could not match the pizza name with the total quantity of pizza sold

For example:

select MAX(sum(quantity)), pizza_name  from order_line group by pizza_name;

it returns following error:

"not a single-group group function"

I guess I could achieve this by using a sub-query, but I have no idea how to do this.

Answer 1

You don't need max for this. If you only want one pizza, then you can use order by and fetch first 1 row only (or something similar such as limit or top ):

select sum(quantity), pizza_name
from order_line
group by pizza_name
order by sum(quantity)
fetch first 1 row only;

Or, if you want all such pizzas, use rank() :

select p.*
from (select sum(quantity) as quantity, pizza_name,
             rank() over (order by sum(quantity) desc) as seqnum
      from order_line
      group by pizza_name
     ) p
where seqnum = 1;

Answer 2

Both of the queries give the same desired result

SELECT PIZZA_NAME,
  SUM(QUANTITY) "Total Quant"
FROM Order_line
GROUP BY PIZZA_NAME
ORDER BY "Total Quant" DESC
FETCH FIRST 1 row only;

SELECT PIZZA_NAME, "Total Quantity" FROM (
SELECT PIZZA_NAME,SUM(QUANTITY) "Total Quantity", RANK() OVER (ORDER BY SUM(QUANTITY) DESC) T FROM Order_line GROUP BY PIZZA_NAME
) query1 where query1.T=1 ;

Answer 3

You group by pizza_name to get sum(quantity) per pizza_name.

Then you aggregate again by using MAX on the quantity sum, but you don't specify which of the three pizza names to have in the result. You need an aggregate function on pizza_name as well, which you don't have. Hence the error.

If you want to use your query, you must apply the appropriate aggregation function on pizza_name, which is KEEP DENSE_RANK FIRST/LAST.

select 
  max(sum(quantity)), 
  max(pizza_name) keep (dense_rank last order by sum(quantity))
from order_line
group by pizza_name;

But on one hand Gordon's queries are more readable in my opinion. And on the other this double aggregation is Oracle specific and not SQL standard. Unexperienced readers may be confused that the query produces one result row in spite of the GROUP BY clause.