sql - 获取最近的非空记录

我正在尝试获取最新记录并找到非NULL匹配项。问题是我的子查询返回多个结果。

资料集

| ID |         DD | SIG_ID |      DCRP |
----------------------------------------
|  1 | 2010-06-01 |      1 |    Expert |
|  2 | 2010-09-01 |      1 |    Expert |
|  3 | 2010-12-01 |      1 |    Expert |
|  4 | 2010-12-01 |      1 | Expert II |
|  5 | 2011-03-01 |      1 | Expert II |
|  6 | 2011-06-01 |      1 |    (null) |
|  7 | 2010-06-01 |      2 |    Senior |
|  8 | 2010-09-01 |      2 |    Senior |
|  9 | 2010-09-01 |      2 |    Senior |
| 10 | 2010-12-01 |      2 | Senior II |
| 11 | 2011-03-01 |      2 |    (null) |
| 12 | 2011-03-01 |      2 |    Senior |
| 13 | 2010-06-01 |      3 |    (null) |
| 14 | 2010-09-01 |      3 |    (null) |
| 15 | 2010-12-01 |      3 |    (null) |

Query

SELECT a.sig_id, a.id,
  CASE
    WHEN b.dcrp IS NULL
      THEN
        (SELECT dcrp
         FROM tbl
         WHERE sig_id = a.sig_id
           AND id < a.id
           AND dcrp IS NOT NULL)
      ELSE b.dcrp
  END AS dcrp
FROM
  (SELECT sig_id, MAX(id) id
  FROM tbl
  GROUP BY sig_id) a
LEFT JOIN
  (SELECT id, dcrp
   FROM tbl
   WHERE dcrp IS NOT NULL) b ON b.id = a.id

所需结果

为每个dcrp获取最新的sig_id：

| ID | DD | SIG_ID | DCRP |
----------------------------------------
| 5 | 2011-03-01 | 1 |专家二|
| 12 | 2011-03-01 | 2 |高级
| 15 | 2010-12-01 | 3 | （空）|

SQL Fiddle

最佳答案

您可以使用以下内容：

;WITH CTE AS
(
    SELECT *, ROW_NUMBER() OVER(PARTITION BY SIG_ID
                                ORDER BY CASE WHEN DCRP IS NOT NULL THEN 0 ELSE 1 END,
                                DD DESC) RN
    FROM tbl
)
SELECT *
FROM CTE
WHERE RN = 1

还有fiddle。