Question

以下是我在Redshift数据库中尝试实现的示例。

我有一个变量current_value，我想要创建一个新列value_desired：

与current_value相同
等于前一个非空值

这听起来很简单，但我还没有找到办法。

row_numb     current_value   value_desired
1
2
3            47              47
4
5            45              45
6
7
8            42              42
9            41              42
10           40              42
11           39              42
12           38              42
13
14           36              36
15
16
17           33              33
18           32              33

我尝试过使用LAG（）函数，但我只能得到之前的值（不是“非null”块中的第一个），这是我的看法：

SELECT *
    , CASE WHEN current_value is not null and LAG(current_value) is null THEN current_value
           WHEN current_value is not null and LAG(current_value) is not null 
            THEN LAG(current_value)
      ELSE NULL END AS value_desired
  FROM test1

非常感谢任何帮助，谢谢。

Answer 1

这是正确的答案，它给出了正确的结果。这里有一些巧妙的技巧，我建议你仔细看看，让我知道需要澄清什么

根据您的问题创建测试数据。

drop table if exists test_table ;
create table test_table (row_num int,current_value int);
insert into test_table(row_num, current_value)
values
  (1,null),
(2,null),
(3,47),
(4,null),
(5,45),
(6,null),
(7,null),
(8 ,42),
(9 ,41),
(10,40  ),
(11,39 ),
(12,38 ),
(13,null),
(14,36),
(15,null),
(16,null),
(17 ,33),
(18,32  )
;

然后运行此代码

SELECT DISTINCT
  j1.row_num,
  CASE WHEN j1.current_value IS NULL
    THEN NULL
  ELSE
    last_value(j2.current_value)
    OVER (
      PARTITION BY j1.row_num
      ORDER BY j2.row_num
      ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING ) END AS value_desired
FROM test_table AS j1
  LEFT JOIN (SELECT
               row_num,
               current_value,
               lag(current_value, 1)
               OVER (
                 ORDER BY row_num ) AS prev_cval
             FROM test_table) AS j2
    ON j1.row_num >= j2.row_num AND j2.current_value IS NOT NULL
       AND j2.prev_cval IS NULL
ORDER BY j1.row_num;

Answer 2

使用FIRST_VALUE（）代替LAG（）

参考：FIRST_VALUE and LAST_VALUE Window Functions

Answer 3

我们的测试数据

DROP TABLE IF EXISTS test_table;
CREATE TABLE test_table (
  row_num       INT,
  current_value INT
);

INSERT INTO test_table (row_num, current_value)
VALUES
  (1, NULL),
  (2, NULL),
  (3, 47),
  (4, NULL),
  (5, 45),
  (6, NULL),
  (7, NULL),
  (8, 42),
  (9, 41),
  (10, 40),
  (11, 39),
  (12, 38),
  (13, NULL),
  (14, 36),
  (15, NULL),
  (16, NULL),
  (17, 33),
  (18, 32);

我们所知道的：

当 current_value 不为空并且 current_value 之前为空时， desired_value 等于 current_value
- 让我们将其称为 first_desired_value ，根据下面的 q_first_desired_value 子查询
- 当 first_desired_value 不为空时，它是我们的 desired_value
- 当current_value不为null时，我们只需要将 first_desired_value 传播到其他行
当 current_value 不为空时，所需的值为前一行（当前行旁边）的最后一个 first_desired_value，并排除可能在之前添加的NULL值帧

鉴于上述情况，查询如下

WITH q_first_desired_value AS
(
    SELECT
      row_num,
      current_value,
      CASE WHEN LAG(current_value, 1)
                OVER (
                  ORDER BY row_num ) IS NULL
        THEN current_value
      ELSE NULL END AS first_desired_value
    FROM test_table
    ORDER BY row_num
)
SELECT
  row_num,
  current_value,
  CASE WHEN first_desired_value IS NOT NULL
    THEN first_desired_value
  WHEN current_value IS NOT NULL
    THEN LAST_VALUE(first_desired_value) IGNORE NULLS
    OVER (
    ORDER BY row_num ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING ) END AS desired_value
FROM q_first_desired_value;

滞后以获得自前一个空值以来的第一个非空值

3 个答案: