BigQueryで日付型の加減を使います

データの作成

WITH purchase_log AS (
  SELECT 'A' AS user_id, '2016-01-30' as last_date, '2016-01-30' as p_date, 'メロン' AS item UNION ALL
  SELECT 'A', 2017-12-30, 2016-01-30, '白菜' UNION ALL
  SELECT 'B', 2016-10-31, 2016-01-30, 'りんご' UNION ALL
  SELECT 'B', 2017-06-30, 2016-01-30, 'りんご' 
)
SELECT  * FROM purchase_log;

user_id	last_date	p_date	item
A	2016-01-30	2017-12-30	メロン
A	2017-12-30	2017-12-31	白菜
B	2016-10-31	2017-08-31	りんご
B	2017-06-30	2018-01-31	りんご

足し算・引き算

SELECT
  user_id,
  last_date,
  -- 足し算
  DATE_ADD(last_date,INTERVAL 10 day ) AS ld_plu,
  -- 引き算
  DATE_SUB(last_date,INTERVAL 10 day) AS ld_minus
FROM
  test1.purchase_log;

user_id	last_date	ld_plu	ld_minus
A	2016-01-30	2016-02-09	2016-01-20
A	2017-12-30	2017-01-09	2017-12-20
B	2016-10-31	2016-11-10	2016-10-21
B	2017-06-30	2017-07-10	2017-06-20

TIMESTAMP_DIFF(usedate, lead1_usedate, DAY)*-1 AS diff_day,

日付型どうしの引き算

SELECT
  user_id,
  last_date,
  p_date,
  -- 日付型どうしの引き算
  -- YEAR, MONTH, DAY
  DATE_DIFF(p_date,last_date, DAY) AS date
FROM
  test1.purchase_log;

user_id	last_date	p_date	date
A	2016-01-30	2017-12-30	700
A	2017-12-30	2017-12-31	1
B	2016-10-31	2017-08-31	304
B	2017-06-30	2018-01-31	215

時刻型どうしの引き算

SELECT
  user_id,
  last_date,
  p_date,
  -- 時刻型の足し算
  TIMESTAMP_ADD(last_date, INTERVAL 10 day)AS after,
  -- 時刻型の引き算
  TIMESTAMP_SUB(last_date, INTERVAL 10 day)AS before,
  -- 時刻型どうしの引き算
  -- DAY, HOUR, MINUTE, SECOND
  TIMESTAMP_DIFF(p_date,last_date, DAY) AS Diff
FROM 
  (
  SELECT
    user_id,
    CAST(last_date AS TIMESTAMP) AS last_date,
    CAST(p_date AS TIMESTAMP) AS p_date
  FROM
    test1.purchase_log
  ) AS a;

| user_id | last_date | p_date | after | before | Diff | | - | - | - | - | | A | 2016-01-30 00:00:00 UTC | 2017-12-30 00:00:00 UTC | 2016-02-09 00:00:00 UTC | 2016-01-20 00:00:00 UTC | 700 | | A | 2017-12-30 00:00:00 UTC | 2017-12-31 00:00:00 UTC | 2017-01-09 00:00:00 UTC | 2017-12-20 00:00:00 UTC | 1 | | B | 2016-10-31 00:00:00 UTC | 2017-08-31 00:00:00 UTC | 2016-11-10 00:00:00 UTC | 2016-10-21 00:00:00 UTC | 304 | | B | 2017-06-30 00:00:00 UTC | 2018-01-31 00:00:00 UTC | 2017-07-10 00:00:00 UTC | 2017-06-20 00:00:00 UTC | 215 |

参考サイト

【SQL】BigQueryで日付型を扱う