检索行中每列的最后一个已知值

Question

不确定问这个问题的正确用词，所以我会分解它。

我有一个表格如下：

date_time | a | b | c

最后 4 行：

15/10/2013 11:45:00 | null   | 'timtim' | 'fred'
15/10/2013 13:00:00 | 'tune' | 'reco'   | null
16/10/2013 12:00:00 | 'abc'  | null     | null
16/10/2013 13:00:00 | null   | 'died'   | null

我如何获取最后一条记录，但该值忽略空值，而是从上一条记录中获取值。

在我提供的示例中，返回的行是

16/10/2013 13:00:00 | 'abc' | 'died' | 'fred'

如您所见，如果列的值为空，那么它将转到具有该列值的最后一条记录并使用该值。

这应该是可能的，我只是想不通。到目前为止我只想到：

select 
    last_value(a) over w a
from test
WINDOW w AS (
    partition by a
    ORDER BY ts asc
    range between current row and unbounded following
    );

但这仅适用于单个列......

Answer 1

行的顺序

“最后一行”和排序顺序需要明确定义。集合（或表）中没有自然顺序。我假设

ORDER BY ts

，其中

ts

是时间戳列。
就像Jorge在他的评论中指出的那样：如果

ts

不是

UNIQUE

，我们需要在

ORDER BY

中添加决胜局以使排序顺序确定。主键很好用。

窗口函数的通用解决方案

要获取每行的结果：

SELECT ts
     , max(a) OVER (PARTITION BY grp_a) AS a
     , max(b) OVER (PARTITION BY grp_b) AS b
     , max(c) OVER (PARTITION BY grp_c) AS c
FROM (
   SELECT *
        , count(a) OVER (ORDER BY ts) AS grp_a
        , count(b) OVER (ORDER BY ts) AS grp_b
        , count(c) OVER (ORDER BY ts) AS grp_c
   FROM tbl
   ) sub;

如何？

聚合函数

count()

在计数时忽略NULL值。用作聚合窗口函数，它根据默认窗口定义计算列的运行计数，即

RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW

。 NULL 值不会增加计数，因此这些行与最后一个非空值属于同一对等组。
在第二个窗口函数中，使用

max()

或 min() 可以轻松提取每组唯一的非空值。

就最后一排

WITH cte AS (
   SELECT *
        , count(a) OVER w AS grp_a
        , count(b) OVER w AS grp_b
        , count(c) OVER w AS grp_c
   FROM   tbl
   WINDOW w AS (ORDER BY ts)
   ) 
SELECT ts
     , max(a) OVER (PARTITION BY grp_a) AS a
     , max(b) OVER (PARTITION BY grp_b) AS b
     , max(c) OVER (PARTITION BY grp_c) AS c
FROM   cte
ORDER  BY ts DESC
LIMIT  1;

最后一行的简单替代方案

SELECT ts
     , COALESCE(a, (SELECT a FROM tbl WHERE a IS NOT NULL ORDER BY ts DESC LIMIT 1)) AS a
     , COALESCE(b, (SELECT b FROM tbl WHERE b IS NOT NULL ORDER BY ts DESC LIMIT 1)) AS b
     , COALESCE(c, (SELECT c FROM tbl WHERE c IS NOT NULL ORDER BY ts DESC LIMIT 1)) AS c
FROM   tbl
ORDER  BY ts DESC
LIMIT  1;

或者：

SELECT (SELECT ts FROM tbl                     ORDER BY ts DESC LIMIT 1) AS ts
     , (SELECT a  FROM tbl WHERE a IS NOT NULL ORDER BY ts DESC LIMIT 1) AS a
     , (SELECT b  FROM tbl WHERE b IS NOT NULL ORDER BY ts DESC LIMIT 1) AS b
     , (SELECT c  FROM tbl WHERE c IS NOT NULL ORDER BY ts DESC LIMIT 1) AS c

小提琴
_旧sqlfiddle

性能

虽然这应该相当快，但如果性能是您的首要要求，请考虑 plpgsql 函数。从最后一行开始，按降序循环，直到所需的每一列都具有非空值。沿着这些思路：

• GROUP BY 和聚合连续数值

Answer 2

这里我创建了一个聚合函数，将列收集到数组中。然后只需删除 NULL 并从每个数组中选择最后一个元素即可。

样本数据

CREATE TABLE T (
    date_time timestamp,
    a text,
    b text,
    c text
);

INSERT INTO T VALUES ('2013-10-15 11:45:00', NULL, 'timtim', 'fred'),
('2013-10-15 13:00:00', 'tune', 'reco', NULL  ),
('2013-10-16 12:00:00', 'abc', NULL, NULL     ),
('2013-10-16 13:00:00', NULL, 'died', NULL    );

解决方案

CREATE AGGREGATE array_accum (anyelement)
(
    sfunc = array_append,
    stype = anyarray,
    initcond = '{}'
);

WITH latest_nonull AS (
    SELECT MAX(date_time) As MaxDateTime, 
           array_remove(array_accum(a), NULL) AS A, 
           array_remove(array_accum(b), NULL) AS B, 
           array_remove(array_accum(c), NULL) AS C
    FROM T
    ORDER BY date_time
)
SELECT MaxDateTime, A[array_upper(A, 1)], B[array_upper(B,1)], C[array_upper(C,1)]
FROM latest_nonull;

结果

     maxdatetime     |  a  |  b   |  c
---------------------+-----+------+------
 2013-10-16 13:00:00 | abc | died | fred
(1 row)

Answer 3

这应该可行，但请记住这是一个丑陋的解决方案

select * from
(select dt from
(select rank() over (order by ctid desc) idx, dt
  from sometable ) cx
where idx = 1) dtz,
(
select a from
(select rank() over (order by ctid desc) idx, a
  from sometable where a is not null ) ax 
where idx = 1) az,
(
select b from
(select rank() over (order by ctid desc) idx, b
  from sometable where b is not null ) bx 
where idx = 1) bz,
(
select c from
(select rank() over (order by ctid desc) idx, c
  from sometable where c is not null ) cx
where idx = 1) cz

在小提琴上查看：http://sqlfiddle.com/#!15/d5940/40

结果将会是

DT                                   A        B      C
October, 16 2013 00:00:00+0000      abc     died    fred