select rows how to query in sql

问题

I have a use case like

a 3
a 4
a 5
a 6
a 5
b 3
b 5
b 3

how to get the output like

a 4
a 5
b 5 
b 3 

choose the hight number for a and b, but just 2 rows

that's the query I wrote now, seems it is not working

SELECT id, barcode, actualsku, inventorycount
FROM (  SELECT pallet.id                             AS id,
           pallet.barcode                        AS barcode,    
           inventoryunit.sku                     AS actualsku, 
           Count(inventoryunit.id)               AS inventorycount 
    FROM   (SELECT * 
        FROM   mft.asset 
        WHERE  container_type = 'PALLET' 
               AND location_type = 'PRIME' :: mft.location_type 
               AND is_deleted = FALSE 
               AND ( attributes ->> 'sku' :: text ) IS NOT NULL) pallet 
           LEFT OUTER JOIN (SELECT * 
                FROM   mft.asset 
                WHERE  asset_type = 'INVENTORYUNIT' :: mft.asset_type 
                       AND is_deleted = FALSE) inventoryunit 
                ON pallet.id = inventoryunit.parent_id 
    GROUP  BY inventoryunit.sku,
          pallet.id,
          pallet.barcode, 
          pallet.attributes ) test
WHERE (SELECT COUNT(*) FROM test as t
    WHERE t.actualsku = test.actualsku
        AND t.inventorycount <= test.inventorycount
        ) <= 2

回答1:

This is typically done using window functions:

select col1, col2 
from (
  select col1, col2, 
         row_number() over (partition by col1 order by col2 desc) as rnk
  from the_table
) t
where rnk <= 2
order by col1, col2;

Online example: http://rextester.com/WKLTSB43296

来源：https://stackoverflow.com/questions/42989516/select-rows-how-to-query-in-sql