TSQL：检查列中至少一个非空值

Question

我想编写一个 TSQL 查询，它独立检查表中的一组列，以查看哪些列至少包含一个非空值。每列的检查应相应地返回 T/F (1/0)。

首先想到的是使用COUNT 聚合函数。由于COUNT(expression) 从结果总数中排除空值，如果COUNT > 0，则存在非空数据。

这似乎有点笨拙，因为它必须计算所有数据。我真的只需要知道每列中是否至少有一个非空值：

    SELECT 
        CAST(CASE WHEN COUNT(t.Column1) > 0 THEN 1 ELSE 0 END AS BIT) AS HasColumn1Data,
        CAST(CASE WHEN COUNT(t.Column2) > 0 THEN 1 ELSE 0 END AS BIT) AS HasColumn2Data,
        CAST(CASE WHEN COUNT(t.Column3) > 0 THEN 1 ELSE 0 END AS BIT) AS HasColumn3Data,
        CAST(CASE WHEN COUNT(t.Column4) > 0 THEN 1 ELSE 0 END AS BIT) AS HasColumn4Data
    FROM dbo.Table AS t
    WHERE t.TimeStamp BETWEEN @StartTimeStamp AND @EndTimeStamp

有什么更好的想法吗？

Answer 1

你可以试试这样的：

;WITH cte AS (
  SELECT * FROM dbo.Table WHERE TimeStamp BETWEEN @StartTimeStamp AND @EndTimeStamp
)
SELECT COUNT(s1.Col1) as Col1, COUNT(s2.Col2) as Col2,
  COUNT(s3.Col3) as Col3, COUNT(s4.Col4) as Col4
FROM
  (SELECT TOP 1 Col1
   FROM cte
   WHERE Col1 IS NOT NULL) s1 CROSS JOIN
  (SELECT TOP 1 Col2
   FROM cte
   WHERE Col2 IS NOT NULL) s2 CROSS JOIN
  (SELECT TOP 1 Col3
   FROM cte
   WHERE Col3 IS NOT NULL) s3 CROSS JOIN
  (SELECT TOP 1 Col4
   FROM cte
   WHERE Col4 IS NOT NULL) s4

如果所有列都不为空，这具有潜在的优势。在这种情况下，表只被扫描到第一个非空行（但这样做 4 次......）。如果所有行的任何（或更糟的是，所有）列都为空，您将获得每列的完整扫描。总而言之，如果您的预期数据确实有值，这可能会很有用。

Answer 2

如果您在列上有索引，以下可能会更快：

select (case when exists (select 1
                          from table t
                          where t.TimeStamp BETWEEN @StartTimeStamp and @EndTimeStamp and
                                column1 is not null
                         )
             then 1 else 0 end) as HasColumn1Data,
       (case when exists (select 1
                          from table t
                          where t.TimeStamp BETWEEN @StartTimeStamp and @EndTimeStamp and
                                column2 is not null
                         )
             then 1 else 0 end) as HasColumn2Data,
       (case when exists (select 1
                          from table t
                          where t.TimeStamp BETWEEN @StartTimeStamp and @EndTimeStamp and
                                column3 is not null
                         )
             then 1 else 0 end) as HasColumn3Data,
       (case when exists (select 1
                          from table t
                          where t.TimeStamp BETWEEN @StartTimeStamp and @EndTimeStamp and
                                column4 is not null
                         )
             then 1 else 0 end) as HasColumn4Data;

如果没有索引，这将是大约 4 次全表扫描（诚然，在第一个非 NULL 值处被截断），因此它可能会比 group by 慢

Answer 3

这可能最终会更麻烦，但使用 EXISTS 而不是 COUNT 可能更理想：

SELECT  CAST(CASE WHEN EXISTS(SELECT * FROM Table t WHERE t.Column1 IS NOT NULL AND t.TimeStamp BETWEEN @StartTimeStamp AND @EndTimeStamp) THEN 1 ELSE 0 END AS BIT) AS HasColumn1Data,
        CAST(CASE WHEN EXISTS(SELECT * FROM Table t WHERE t.Column2 IS NOT NULL AND t.TimeStamp BETWEEN @StartTimeStamp AND @EndTimeStamp) THEN 1 ELSE 0 END AS BIT) AS HasColumn2Data,
        CAST(CASE WHEN EXISTS(SELECT * FROM Table t WHERE t.Column3 IS NOT NULL AND t.TimeStamp BETWEEN @StartTimeStamp AND @EndTimeStamp) THEN 1 ELSE 0 END AS BIT) AS HasColumn3Data,
        CAST(CASE WHEN EXISTS(SELECT * FROM Table t WHERE t.Column4 IS NOT NULL AND t.TimeStamp BETWEEN @StartTimeStamp AND @EndTimeStamp) THEN 1 ELSE 0 END AS BIT) AS HasColumn4Data

Answer 4

我会将查询转为仅生成字段名称以及它们是否为非空：

SELECT
  'COL1' AS column_name,
  CONVERT( BIT, COUNT( 1 ) ) AS is_not_entirely_null
FROM
  foo
WHERE
  column1 IS NOT NULL
UNION
SELECT
  'COL2' AS column_name,
  CONVERT( BIT, COUNT( 1 ) ) AS is_not_entirely_null
FROM
  foo
WHERE
  column2 IS NOT NULL

...

顺便说一句，您应该能够使用以下内容自动生成上述查询：

SELECT
  'SELECT ''' + c.name + ''' AS column_name, CONVERT( BIT, COUNT( 1 ) ) AS is_not_entirely_null FROM ' + t.name + ' WHERE ' + c.name + ' IS NOT NULL UNION'
FROM 
  sysobjects AS t,
  syscolumns AS c
WHERE
  t.name = 'foo' AND
  c.id = t.id

Answer 5

你可以使用这个查询

SELECT
max(CASE WHEN t.Column1 IS NULL THEN 0 ELSE 1 END ) AS HasColumn1Data,
max(CASE WHEN t.Column2 IS NULL THEN 0 ELSE 1 END ) AS HasColumn2Data,
max(CASE WHEN t.Column3 IS NULL THEN 0 ELSE 1 END ) AS HasColumn3Data,
max(CASE WHEN t.Column4 IS NULL THEN 0 ELSE 1 END ) AS HasColumn4Data,
FROM dbo.Table AS t
WHERE t.TimeStamp BETWEEN @StartTimeStamp AND @EndTimeStamp