Question

使用下表：

A | B    | C    | ts
--+------+------+------------------
1 | null | null | 2016-06-15 10:00
4 | null | null | 2016-06-15 11:00 
4 |    9 | null | 2016-06-15 12:00
5 |    1 |    7 | 2016-06-15 13:00

如何在N行的运行窗口中选择每列的第一个非空值？ “第一个”由列ts中的时间戳顺序定义。查询上表将导致：

A | B | C
--+---+---
1 | 9 | 7

Answer 1

窗口函数first_value()允许一个相当简短和优雅的解决方案：

SELECT first_value(a) OVER (ORDER BY a IS NULL, ts) AS a
     , first_value(b) OVER (ORDER BY b IS NULL, ts) AS b
     , first_value(c) OVER (ORDER BY c IS NULL, ts) AS c
FROM   t
LIMIT  1;

a IS NULL评估为TRUE或FALSE。 FALSE之前TRUE排序。这样，非空值首先出现。接下来按ts排序（您评论的时间戳列），然后您就可以在一个SELECT中找到它。

如果Postgres支持IGNORE NULLS，这会更简单。 The manual:

SQL标准定义了RESPECT NULLS或IGNORE NULLS选项 lead，lag，first_value，last_value和nth_value。这不是在PostgreSQL中实现：行为总是与标准的默认值，即RESPECT NULLS。

该领域关于标准SQL的少数遗漏之一。

db＆lt;＆gt;小提琴here
SQL Fiddle.

Answer 2

您应该定义一个订单（主键或其他内容）以获取FIRST非空值。所以我使用ID列来排序表格中的行。

select 
(select A from t where A is not null ORDER BY id LIMIT 1),
(select b from t where b is not null ORDER BY id LIMIT 1),
(select c from t where c is not null ORDER BY id LIMIT 1)

SQLFiddle demo

Answer 3

您可以使用window functions执行此操作。我已将结果分为两部分，然后将此分区用于row_number window function

具有空值
拥有有效的有效

然后，使用基本case来获取row_number为1并且其中包含not null值的

SQLFIDDLE

SELECT
    max ( CASE
            WHEN a_row_num = 1 AND a IS NOT NULL THEN a
          END ) AS A,
    max ( CASE
            WHEN b_row_num = 1 AND B IS NOT NULL THEN B
          END ) AS B,
    max ( CASE
            WHEN c_row_num = 1 AND C IS NOT NULL THEN C
          END ) AS C
FROM
    (
      SELECT
          a,
          row_number ( ) over ( partition BY a IS NULL ORDER BY ID ) a_row_num,
          b,
          row_number ( ) over ( partition BY b IS NULL ORDER BY ID ) b_row_num,
          c,
          row_number ( ) over ( partition BY c IS NULL ORDER BY ID ) c_row_num
      FROM
          test
    ) AS sub_query

<强>输出：

| A | B | C |
|---|---|---|
| 1 | 9 | 7 |

注意：我添加了id字段，这有助于了解哪些记录首次插入，我们在窗口函数中按升序使用

Answer 4

不确定我是否正确地提出了问题因为它看起来很简单。
试试这个查询
SQL小提琴：http://sqlfiddle.com/#!11/ac585/8

WITH t0 AS
(
    SELECT A FROM
    TableName t0
    WHERE (A IS NOT NULL)
    ORDER BY ID ASC
    LIMIT 1
),

t1 AS
(
    SELECT B FROM
    TableName
    WHERE (B IS NOT NULL)
    ORDER BY ID ASC
    LIMIT 1
),

t2 AS

(
    SELECT C FROM
    TableName
    WHERE (C IS NOT NULL)
    ORDER BY ID ASC
    LIMIT 1
)

SELECT t0.A, t1.B, t2.C
FROM
t0
JOIN t1 ON 1=1
JOIN t2 ON 1=1

在稀疏表中选择一行第一个非空值

4 个答案: