SQL按任意顺序查找第一个非空值

只要做:

SELECT TOP 1 Value
FROM mytable
WHERE Value IS NOT NULL
ORDER BY ID

获得 最后的 非空值切换顺序:

ORDER BY ID DESC

SELECT (SELECT TOP 1 Value1 FROM mytable 
        WHERE Value1 IS NOT NULL ORDER BY ID) AS min_Value1,
       (SELECT TOP 1 Value2 FROM mytable 
        WHERE Value2 IS NOT NULL ORDER BY ID) AS min_Value2

Demo here

对于那些需要在特定时间内这样做的人 大容量 ,这将成为一个问题和密集的,因为它可以转化为后端上的大量单个表点击。

我有一个使用窗口函数的解决方案,可以解决大数据/分区的问题。

GroupID | ID | Value1 | Value2
--------------------------
1       |  1 |  NULL  |   AA
1       |  2 |   B    |  NULL
1       |  3 |   C    |   CC
1       |  4 |  NULL  |  NULL
2       |  5 |   E    |   EE
2       |  6 |   F    |  NULL
2       |  7 |  NULL  |   GG
2       |  8 |  NULL  |  NULL

对于每个GroupID值

GroupID | FirstValue1 | FirstValue2 | LastValue1 | LastValue2
-------------------------------------------------------------
1       | B           | AA          | C          | CC
2       | E           | EE          | F          | GG

如果我将前1个逻辑应用于缩放,并且我有很多行,那么它将返回以执行大量的单个表查询。然而,如果我使用窗口函数,我可以让它评估内存中的数据,以更有效地获取数据,尤其是在需要分区和大量数据时。

以下是解决方案:

SELECT
  GroupID,
  FirstValue1,
  FirstValue2,
  LastValue1,
  LastValue2
FROM
  (
  SELECT
    GroupID,
    ID, Value1, Value2,
    -- Sets a rank so we can reduce to 1 row per GroupID in outer query
    DENSE_RANK() OVER (
      PARTITION BY GroupID
      ORDER BY ID
    ) IDRank,
    FIRST_VALUE(Value1) OVER (
      PARTITION BY GroupID
      ORDER BY
        CASE WHEN Value1 IS NULL THEN 2 ELSE 1 END, -- Prioritize non-null Value1
        ID                                          -- And ascending ID
    ) FirstValue1,
    FIRST_VALUE(Value2) OVER (
      PARTITION BY GroupID
      ORDER BY
        CASE WHEN Value2 IS NULL THEN 2 ELSE 1 END, -- Prioritize non-null Value2
        ID                                          -- And ascending ID
    ) FirstValue2,
    FIRST_VALUE(Value1) OVER (
      PARTITION BY GroupID
      ORDER BY
        CASE WHEN Value1 IS NULL THEN 2 ELSE 1 END, -- Prioritize non-null Value1
        ID DESC                                     -- And descending ID
    ) LastValue1,
    FIRST_VALUE(Value2) OVER (
      PARTITION BY GroupID
      ORDER BY
        CASE WHEN Value2 IS NULL THEN 2 ELSE 1 END, -- Prioritize non-null Value2
        ID DESC                                     -- And descending ID
    ) LastValue2
  FROM MY_TABLE
  ) BestValues
WHERE IDRank = 1  --Ensures we get only one row per GroupID
ORDER BY GroupID

解释-第一个_值在每个不同的GroupID内求值(因为我们按GroupID设置分区)。在该窗口中,它计算该GroupID的行中具有非空值的情况,然后按ID排序(升序或降序)。 由于它是一个窗口函数,这将值放在原始粒度的记录级别上,因此我们还添加了一个稠密的_秩来进行秩,并使我们能够在外部查询中,将每个GroupID降到一行。

p、 如果您想自己运行它,请在查询中包含以下内容,以将所需的样本数据生成到CTE中的“MY_表”中。

WITH MY_TABLE AS (
  SELECT 0 as GroupID, 0 AS ID, CAST(NULL AS VARCHAR(10)) as Value1, CAST(NULL AS VARCHAR(10)) as Value2 WHERE 0=1 UNION ALL --Dummy row to set types

  SELECT 1 AS GroupID, 1 AS ID, NULL AS Value1, 'AA' AS Value2 UNION ALL
  SELECT 1 AS GroupID, 2 AS ID, 'B'  AS Value1, NULL AS Value2 UNION ALL
  SELECT 1 AS GroupID, 3 AS ID, 'C'  AS Value1, 'CC' AS Value2 UNION ALL
  SELECT 1 AS GroupID, 4 AS ID, NULL AS Value1, NULL AS Value2 UNION ALL
  SELECT 2 AS GroupID, 5 AS ID, 'E'  AS Value1, 'EE' AS Value2 UNION ALL
  SELECT 2 AS GroupID, 6 AS ID, 'F'  AS Value1, NULL AS Value2 UNION ALL
  SELECT 2 AS GroupID, 7 AS ID, NULL AS Value1, NULL AS Value2 UNION ALL
  SELECT 2 AS GroupID, 8 AS ID, NULL AS Value1, 'GG' AS Value2
)

Coalesce function

你的问题最终会是

SELECT COALESCE(Value ORDER BY ID) FROM MY_TABLE

Example here

SELECT distinct TOP 1 Value
FROM mytable
WHERE Value IS NOT NULL
ORDER BY ID desc