BigQuery：加入 2 个表，但只根据日期列选择行

Question

我有 2 table

第一个tabletable_new_data像

date    type  data
2022-01 t1    0
2022-03 t2    1
2021-08 t1    1

第二个tabletable_old_data就像

date    type  data
2021-10 t1    2
2022-04 t2    3
2021-07 t1    4
2021-06 t1    5

我想要 sql 代码片段 table_new_data LEFT JOIN table_old_data 并产生以下结果。

new_date type new_data old_date  old_data
2022-01  t1   0        2021-10   2
2022-03  t2   1        null      null
2021-08  t1   1        2021-07   4

请注意，

仅连接具有相同 type
对于 table_new_data 中的每一行，仅与 table_old_data 中具有 最接近的前一行 date 的行连接。例如，对于 table_new_data 中的 2021-08 t1 1，我们只想加入 table_old_data 中的 2021-07 t1 4。

date 在 YYYY-MM 中。

Answer 1

见下文...考虑边缘情况。


--  ---------------------------------------------------------------------------------
--  create dummy NEW table 
--  ---------------------------------------------------------------------------------
WITH
  table_new_data AS (
  SELECT
    '2022-01' AS date,
    't1' AS type,
    0 AS DATA
  UNION ALL
  SELECT
    '2022-03' AS date,
    't2' AS type,
    1 AS DATA
  UNION ALL
  SELECT
    '2021-08' AS date,
    't1' AS type,
    1 AS DATA ),
--  ---------------------------------------------------------------------------------
--  create dummy OLD table 
--  ---------------------------------------------------------------------------------
  table_old_data AS (
  SELECT
    '2021-10' AS date,
    't1' AS type,
    2 AS DATA
  UNION ALL
  SELECT
    '2022-04' AS date,
    't2' AS type,
    3 AS DATA
  UNION ALL
  SELECT
    '2021-07' AS date,
    't1' AS type,
    4 AS DATA
  UNION ALL
  SELECT
    '2021-06' AS date,
    't1' AS type,
    5 AS DATA),
--  ---------------------------------------------------------------------------------
--  create joined tables based on dates from old table being LOWER (may need <=??)
--  create order = ROW_NUMBER() function to see which date is closest from old table
--  make sure to test on edge cases where dates are the same or equal to
--  ---------------------------------------------------------------------------------
  ordered AS (
  SELECT
    nd.date AS new_date,
    nd.type,
    nd.DATA AS new_data,
    od.date AS old_date,
    od.DATA AS old_data,
    ROW_NUMBER() OVER(PARTITION BY nd.type, nd.date ORDER BY nd.date ) AS rn
  FROM
    table_new_data nd
  LEFT JOIN
    table_old_data od
  ON
    nd.type = od.type
    AND od.date < nd.date )
--  ---------------------------------------------------------------------------------
--  final table to reproduce desired output in question
--  ---------------------------------------------------------------------------------
SELECT
  * EXCEPT(rn)
FROM
  ordered
WHERE
  rn = 1

输出：

BigQuery：加入 2 个表，但只根据日期列选择行

BigQuery: join 2 tables but only choosing rows based on date column

sql

google-bigquery