如何选择组中每列的最后一组非 NULL 值？

Question

我正在使用 SQL Server 2016 并且我使用的数据具有以下形式。

CREATE TABLE #tab (cat CHAR(1), t CHAR(2), val1 INT, val2 CHAR(1));

INSERT INTO #tab VALUES 
    ('A','Q1',2,NULL),('A','Q2',NULL,'P'),('A','Q3',1,NULL),('A','Q3',NULL,NULL),
    ('B','Q1',5,NULL),('B','Q2',NULL,'P'),('B','Q3',NULL,'C'),('B','Q3',10,NULL);

SELECT *
FROM    #tab;

我想获取列上的最后一个非空值，val1并按val2分组cat和排序t。我寻求的结果是

cat  val1 val2
A    1    P
B    10   C

我最接近的是使用LAST_VALUE而忽略ORDER BY哪个不起作用，因为我需要有序的最后一个非空值。

SELECT DISTINCT 
        cat, 
        LAST_VALUE(val1) OVER(PARTITION BY cat ORDER BY (SELECT NULL) ) AS val1,
        LAST_VALUE(val2) OVER(PARTITION BY cat ORDER BY (SELECT NULL) ) AS val2
FROM    #tab

cat  val1 val2
A    NULL NULL
B    10   NULL

实际表有更多列cat（日期和字符串列）和更多 val 列（日期、字符串和数字列）来选择最后一个非空值。

任何想法如何进行此选择。

Answer 1

使用Itzik Ben Gan 的The Last non NULL Puzzle 中的串联技术，对于您的示例表和列数据类型，将看起来像这样。

select T.cat,
       cast(substring(
                     max(cast(T.t as binary(2)) + cast(T.val1 as binary(4))),
                     3,
                     4
                     ) as int),
       cast(substring(
                     max(cast(T.t as binary(2)) + cast(T.val2 as binary(1))),
                     3,
                     1
                     ) as char(1))
from #tab as T
group by T.cat;

编写此查询的另一种方法是将步骤划分为 CTE，以便更好地显示正在发生的事情。它给出了与上面的查询完全相同的执行计划。

with C1 as
(
  -- Concatenate the ordering column with the value column
  select T.cat,
        cast(T.t as binary(2)) + cast(T.val1 as binary(4)) as val1,
        cast(T.t as binary(2)) + cast(T.val2 as binary(1)) as val2
  from #tab as T
),
C2 as
(
  -- Get the max concatenated value per group
  select C1.cat,
         max(C1.val1) as val1,
         max(C1.val2) as val2
  from C1
  group by C1.cat
)
-- Extract the value from the concatenated column
select C2.cat,
       cast(substring(C2.val1, 3, 4) as int) as val1,
       cast(substring(C2.val2, 3, 1) as char(1)) as val2
from C2;

此解决方案使用了将空值与某些内容连接会导致空值的事实。SET CONCAT_NULL_YIELDS_NULL (Transact-SQL)

Answer 2

只需在分区中添加对 NULL 的检查即可

SELECT DISTINCT 
        cat, 
        FIRST_VALUE(val1) OVER(PARTITION BY cat ORDER BY CASE WHEN val1 is NULL then 0 else 1 END DESC, t desc) AS val1,
        FIRST_VALUE(val2) OVER(PARTITION BY cat ORDER BY CASE WHEN val2 is NULL then 0 else 1 END DESC, t desc) AS val2
FROM    #tab