SQL 过滤与 COALESCE 或 window function

Question

I have a table with the following simplified data:

PK    FK   Type
----------------
1     11    A
2     11    B
3     12    B
4     13    C
5     13    D
6     14    D

And I want a result set of:

PK    FK   Type
---------------
1     11    A
3     12    B
4     13    C
6     14    D

So if a given FK value has a Type A, don't give me any rows with B, C, or D. Or if the value has a Type B, then filter out C and D, etc. This feels like I need to apply a window function or coalesce, but I'm not sure how to do it.

Answer 1

Use:

SELECT x.* 
  FROM (SELECT s.*, 
               ROW_NUMBER() OVER(PARTITION BY s.fk
                                     ORDER BY s.pk) rnk
          FROM YOUR_TABLE s) x
 WHERE x.rnk = 1

...or using CTE (no performance difference):

WITH example AS (
  SELECT s.*, 
         ROW_NUMBER() OVER(PARTITION BY s.fk
                               ORDER BY s.pk) rnk
    FROM sample s)
SELECT x.* 
  FROM example x
 WHERE x.rnk = 1

Proof:

WITH sample AS (
  SELECT 1 AS PK, 11 AS FK, 'A' AS [TYPE]
  UNION ALL
  SELECT 2 AS PK, 11 AS FK, 'B' AS [TYPE]
  UNION ALL
  SELECT 3 AS PK, 12 AS FK, 'B' AS [TYPE]
  UNION ALL
  SELECT 4 AS PK, 13 AS FK, 'C' AS [TYPE]
  UNION ALL
  SELECT 5 AS PK, 13 AS FK, 'D' AS [TYPE]
  UNION ALL
  SELECT 6 AS PK, 14 AS FK, 'D' AS [TYPE])
SELECT x.* 
  FROM (SELECT s.*, ROW_NUMBER() OVER(PARTITION BY s.fk
                                          ORDER BY s.pk) rnk
          FROM sample s) x
 WHERE x.rnk = 1

Result:

pk   fk   type
----------------
1    11   A
3    12   B
4    13   C
6    14   D

Answer 2

So, for each FK value, you want the minimal PK value that matches, and the corresponding type for the PK? As ever, do it in steps.

Which are the PK values we want?

SELECT FK, MIN(PK) AS PK
  FROM SimplifiedData
 GROUP BY FK

How to get the corresponding rows - why, join with the main table, of course:

SELECT S.PK, S.FK, S.Type
  FROM SimplifiedData AS S
  JOIN (SELECT FK, MIN(PK) AS PK
          FROM SimplifiedData
         GROUP BY FK
       ) AS T ON S.PK = T.PK
 ORDER BY S.FK;