Computing a moving maximum in BigQuery

Question

Given a BigQuery table with some ordering, and some numbers, I\'d like to compute a \"moving maximum\" of the numbers -- similar to a moving average, but for a maximum inste

Answer 1

There's an example creating a moving using window function in the docs here.

Quoting:

The following example calculates a moving average of the values in the current row and the row preceding it. The window frame comprises two rows that move with the current row.

#legacySQL
SELECT
  name,
  value,
  AVG(value)
    OVER (ORDER BY value
          ROWS BETWEEN 1 PRECEDING AND CURRENT ROW)
    AS MovingAverage
FROM
  (SELECT "a" AS name, 0 AS value),
  (SELECT "b" AS name, 1 AS value),
  (SELECT "c" AS name, 2 AS value),
  (SELECT "d" AS name, 3 AS value),
  (SELECT "e" AS name, 4 AS value);

Answer 2

A trick I'm using for rolling windows: CROSS JOIN with a table of numbers. In this case, to have a moving window of 3 years, I cross join with the numbers 0,1,2. Then you can create an id for each group (ending_at_year==year-i) and group by that.

SELECT ending_at_year, MAX(mean_temp) max_temp, COUNT(DISTINCT year) c
FROM 
(
 SELECT mean_temp, year-i ending_at_year, year
 FROM [publicdata:samples.gsod] a
 CROSS JOIN 
  (SELECT i FROM [fh-bigquery:public_dump.numbers_255] WHERE i<3) b
 WHERE station_number=722860
)
GROUP BY ending_at_year
HAVING c=3
ORDER BY ending_at_year;

Answer 3

I have another way to do the thing you are trying to achieve. See query below

SELECT word, max(words)
FROM 
  (SELECT word,
    word_count AS words
  FROM [publicdata:samples.shakespeare]
  WHERE corpus = 'macbeth'), 
  (SELECT word,
    LEAD(word_count, 1) OVER (ORDER BY word) AS words
  FROM [publicdata:samples.shakespeare]
  WHERE corpus = 'macbeth'), 
  (SELECT word,
    LEAD(word_count, 2) OVER (ORDER BY word) AS words
  FROM [publicdata:samples.shakespeare]
  WHERE corpus = 'macbeth')
group by word order by word

You can try it and compare performance with your approach (I didn't try that)