SQL select a row by highest value from a group

Question

I'm trying to select the highest value(calculated) of a group with distinct selection of other columns

according to the table-data below, i want to select the rows, which have the highest amount (Qty-Plan) and a distinct selection of Len and Wid

Table data is as follow

+-----------+-----------+---------+---------+------------+---------+
|   Ident   |   Name    | Len     |   Wid   |      Qty   |    Plan |
+-----------+-----------+---------+---------+------------+---------+
|  12345    | Name1     | 1500    |    1000 |         20 |       5 |
|  23456    | Name1     | 1500    |    1000 |         30 |      13 |
|  34567    | Name1     | 2500    |    1000 |         10 |       2 |
|  45678    | Name1     | 2500    |    1000 |         10 |       4 |
|  56789    | Name1     | 1500    |    1200 |         20 |       3 |
|  00001    | Name2     | 1500    |    1200 |         10 |       6 |
|  00002    | Name2     | 1500    |    1200 |         20 |       7 |
|  00003    | Name3     | 1500    |    1200 |         30 |       5 |
|  00004    | Name3     | 1500    |    1200 |         40 |       4 |
+-----------+-----------+---------+---------+------------+---------+

with my query i cant erase the "lower" values:

select a.Ident ,a.Name, a.Len,a.Wid, a.Qnt-a.Plan as Amount
from table a
join (select ident, max(Qnt - Plan) Amount
      from table
      where Name = 'Name1'
      group by Ident, Len, Wid) b
  on b.Ident = a.Ident and b.Amount = a.Qnt-a.Plan
order by Amount desc

off-topic question: why i cant use -> where b.Amount = a.Amount (he does not know a.Amount ) ???

my desired select should look like:

+-----------+-----------+---------+---------+------------+
|   Ident   |   Name    | Len     |   Wid   |   Amount   |
+-----------+-----------+---------+---------+------------+
|  56789    | Name1     | 1500    |    1200 |         18 |
|  23456    | Name1     | 1500    |    1000 |         17 |
|  34567    | Name1     | 2500    |    1000 |          8 |
+-----------+-----------+---------+---------+------------+

thanks a lot in advance

Answer 1

It's not clear what kind of database you're using, but this solution should work on any DB:

SELECT tab.Ident,
       tab.Name,
       tab.Len,
       tab.Wid,
       (tab.Qty - tab.Plan) AS Amount
FROM   (SELECT   Name,
                 Len,
                 Wid,
                 MAX(Qty-Plan) AS Amount
        FROM     my_table
        GROUP BY Name,
                 Len,
                 Wid
       ) AS grouped
JOIN   my_table tab
  ON   grouped.Name = tab.Name
 AND   grouped.Len = tab.Len
 AND   grouped.Wid = tab.Wid
 AND   grouped.Amount = (tab.Qty - tab.Plan)
 AND   tab.Name = 'Name1'

Answer 2

Another approach, using window functions to simplify things:

SELECT ident, name, len, wid, qnt - [plan] AS amount
FROM  (SELECT *, row_number() OVER (PARTITION BY len, wid ORDER BY qnt - [plan] DESC) AS rn
       FROM test WHERE name = 'Name1') AS sq
WHERE rn = 1
ORDER BY amount DESC;

SQL Fiddle example .