Bigquery Lag 导航功能忽略空值

Question

我有一张表如下：

WITH
  table1 AS (
  SELECT
    TIMESTAMP '2016-10-18 2:51:45' AS ts,
    'AB4' AS packId,
    FALSE AS stat,
    'x' AS position
  UNION ALL
  SELECT
    TIMESTAMP '2016-10-18 2:54:11',
    'AB4',
    FALSE,
    'x'
  UNION ALL
  SELECT
    TIMESTAMP '2016-10-18 3:01:17',
    'AB4',
    FALSE,
    'x'
  UNION ALL
  SELECT
    TIMESTAMP '2016-10-18 3:05:42',
    'AB4',
    TRUE,
    NULL
  UNION ALL
  SELECT
    TIMESTAMP '2016-10-18 3:06:24',
    'AB4',
    FALSE,
    'x'
  UNION ALL
  SELECT
    TIMESTAMP '2016-10-18 3:06:36',
    'AB4',
    TRUE,
    'y'
  UNION ALL
  SELECT
    TIMESTAMP '2016-10-18 3:07:41',
    'AB4',
    FALSE,
    'x')
SELECT
  ts,
  packId,
  stat,
  position,
  LAG(position) OVER prev_pos AS previous_position1,
  LAG(position,2) OVER prev_pos AS previous_position2
FROM
  table1
WINDOW
  prev_pos AS (
  PARTITION BY
    packId
  ORDER BY
    ts)

结果如下所示：

我想获取不为 null 的先前值（从列位置），如结果表中所示，我希望这些 null 值是先前的非 null 值，在本例中这两个值都是“x”。（有数千行具有空位置，因此当前行的最后一个非空位置可能是其之前的数千行）

总之：如何忽略bigquery中Lag的空值？

Answer 1

以下是 BigQuery 标准 SQL

WITH table1 AS (
  SELECT TIMESTAMP '2016-10-18 2:51:45' AS ts, 'AB4' AS packId, FALSE AS stat, 'x3' AS position UNION ALL
  SELECT TIMESTAMP '2016-10-18 2:54:11', 'AB4', FALSE, 'x2' UNION ALL
  SELECT TIMESTAMP '2016-10-18 3:01:17', 'AB4', FALSE, 'x1' UNION ALL
  SELECT TIMESTAMP '2016-10-18 3:05:42', 'AB4', TRUE, NULL UNION ALL
  SELECT TIMESTAMP '2016-10-18 3:06:24', 'AB4', FALSE, 'x' UNION ALL
  SELECT TIMESTAMP '2016-10-18 3:06:36', 'AB4', TRUE, 'y' UNION ALL
  SELECT TIMESTAMP '2016-10-18 3:07:41', 'AB4', FALSE, 'x'
)
SELECT
  ts,
  packId,
  stat,
  position,
  NTH_VALUE(position, 1 IGNORE NULLS) OVER prev_pos AS previous_position1,
  NTH_VALUE(position, 2 IGNORE NULLS) OVER prev_pos AS previous_position2
FROM  table1
WINDOW prev_pos AS 
  (PARTITION BY packId ORDER BY ts DESC ROWS BETWEEN 1 FOLLOWING and UNBOUNDED FOLLOWING)
-- ORDER BY ts     

带输出

正如您所看到的，不需要子查询 - 对原始查询的唯一更改是

1. 分别替换LAG(position)和LAG(position,2)为 -NTH_VALUE(position, 1 IGNORE NULLS)和NTH_VALUE(position, 2 IGNORE NULLS)和

2. prev_pos向窗口添加适当的框架子句