MySql：查找最大SUM值？

Question

I would like to find out the highest quantity of the item of each category.

I have a table named "products" as shown below :

+----------+-------+-----+
| Category | Item  | Qty |
+----------+-------+-----+
| A        | A100  |  15 |
| A        | A100  |   7 |
| A        | A101  |   3 |
| A        | A101  |   9 |
| B        | B100  |   6 |
| B        | B100  |   7 |
| B        | B101  |   8 |
| B        | B101  |  13 |
+----------+-------+-----+

After I execute the query

SELECT Category, Item, SUM(Qty) as Quantity FROM products GROUP BY Category,Item;

I got the results as below

+----------+-------+----------+
| Category | Item  | Quantity |
+----------+-------+----------+
| A        | A100  |       22 |
| A        | A101  |       12 |
| B        | B100  |       13 |
| B        | B101  |       21 |
+----------+-------+----------+

Now I want to find the highest quantity of the item in each category, which I want to have my output as follows :

+----------+-------+----------+
| Category | Item  | Quantity |
+----------+-------+----------+
| A        | A100  |       22 |
| B        | B101  |       21 |
+----------+-------+----------+

how can I do that? I have tried the query below but I only got 'OK' as the result without giving me any output table.

select Category, Item, max(Qty) as Quantity from products group by OD_type  having Quantity = (select sum(Qty) from products group by Item);

How can I get my desired output as shown in the table above? I am sorry if this is not a good question or my question is not clear enough. Any help will be appreciated, Thank you!

Answer 1

This would be easier to use a view or a common-table-expression (if mysql would support it).

SELECT t.category, t.item, t.quantity
FROM (
    SELECT Category, Item, SUM(Qty) Quantity 
    FROM products 
    GROUP BY Category, Item
  ) t JOIN (
    SELECT category, max(quantity) Quantity
    FROM (
      SELECT Category, Item, SUM(Qty) Quantity 
      FROM products 
      GROUP BY Category,Item
    ) t
    GROUP BY category
  ) t2 ON t.Category = t2.Category and t.quantity = t2.quantity

• SQL Fiddle Demo

The basic idea is to get the sum grouped by each category and item, then to get the max(sum) by each category, and then join back to the original sum query...

Answer 2

you need to use it as subquery , in case there are two items with same max value, you will get only one of them.

SELECT Category, Item, Max(Quantity) as MaxVal
FROM
(
SELECT Category, Item, SUM(Qty) as Quantity 
FROM products 
GROUP BY Category,Item
) T
GROUP BY Category

Answer 3

This looks ugly and I'm seeing if mysql has a better way... but I believe it works...

SELECT A.category, A.Item, A.Quantity
FROM (SELECT Category, Item, SUM(Qty) as Quantity 
      FROM products 
      GROUP BY Category,Item) A
INNER JOIN (Select Max(B.quantity) Quantity, B.Category 
            FROM 
              (SELECT Category, SUM(Qty) as Quantity 
               FROM products 
               GROUP BY Category, Item) B
            GROUP BY Category) C
  ON A.Category=C.Category
 and A.Quantity = C.Quantity

I want to use a window set but it appears mysql doesn't support them. And since we have to aggregate the data first before we can get the max, the inner select is necessary.. and I wanted to use a CTE to improve readability, but MySQL doesn't support them either...

I would probably create tables A, B as a View and call the view to make this easier to read long run.