BigQuery：如何通过窗口函数合并 HLL 草图 （在滚动窗口上计算不同值）

sql
WITH
  hll_per_window AS (
    SELECT
      window_start,
      window_end,
      APPROX_TOP_COUNT(value, 1000) AS hll
    FROM
      (SELECT
        value,
        TIMESTAMP_TRUNC(event_timestamp, HOUR) AS window_start,
        TIMESTAMP_ADD(TIMESTAMP_TRUNC(event_timestamp, HOUR), INTERVAL 1 HOUR) AS window_end
      FROM
        <code>project.dataset.table</code>
      )
    GROUP BY
      window_start,
      window_end
  ),
  merged_hll AS (
    SELECT
      APPROX_TOP_COUNT.MERGE(hll) AS merged_hll
    FROM
      hll_per_window
  )
SELECT
  APPROX_COUNT_DISTINCT(merged_hll) AS distinct_values_count
FROM
  merged_hll