在SQL Server中使用优先级条件进行过滤

Question

我正在尝试过滤表1：

 Table1

 Region             mCode   pCode
 Europa               AD    E
 Rest of the world    AD    O
 East Europa          AE    O
 Outside              AE    L
 Rest of the world    AE    E
 Asia                 AF    O
 North America        AG    D
 Rest of the world    AG    L
 North America        AI    D
 Rest of the world    AI    L
 America              AI    L

pCode（D，L，E，O）有四个不同的值，可以重复mCode。

根据以下情况，我仅需要获得mCode具有优先级为pCode的那些行：

Highest priority     pCode = D
Second priority      pCode = L 
Third priority       pCode = E 
last priority        pCode = O 

例如，mCode'AE'出现在3行中，而pCode在不同的行中分别是'O'，'L'和'E'。根据pCode优先级，结果将显示pCode为第二优先级“ L”的行，因为没有“ AE”的行具有高于“ L”的优先级。其余的行并不重要。

所需的结果将mCode作为唯一值：

 Region            mCode    pCode
 Europa            AD       E
 Outside           AE       L
 Asia              AF       O
 North America     AG       D
 North America     AI       D

Answer 1

您可以使用row_number（）为每个mCode分配优先级，然后仅过滤优先级最高的优先级

select *
from
(
    select *, p = row_number() over (partition by mCode
                                          order by case when pCode = 'D' then 1
                                                        when pCode = 'L' then 2
                                                        when pCode = 'E' then 3
                                                        when pCode = 'O' then 4
                                                        end)
    from   Table1
) d
where d.p = 1

Answer 2

将带有ROW_NUMBER()表达式的窗口函数CASE使用为>]

SELECT Region, mCode, pCode
FROM
(
SELECT *, 
ROW_NUMBER() OVER(PARTITION BY mCode ORDER BY CASE pCode WHEN 'D' THEN 0
                    WHEN 'L' THEN 1
                    WHEN 'E' THEN 2
                    WHEN 'O' THEN 3
                    END) RN
FROM
(
  VALUES
  ('Europa'               ,'AD',    'E'),
  ('Rest of the world'    ,'AD',    'O'),
  ('East Europa'          ,'AE',    'O'),
  ('Outside'              ,'AE',    'L'),
  ('Rest of the world'    ,'AE',    'E'),
  ('Asia'                 ,'AF',    'O'),
  ('North America'        ,'AG',    'D'),
  ('Rest of the world'    ,'AG',    'L'),
  ('North America'        ,'AI',    'D'),
  ('Rest of the world'    ,'AI',    'L'),
  ('America'              ,'AI',    'L')
) T(Region, mCode, pCode)
) TT
WHERE RN = 1;
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