如何按较小的最近日期连接表

Question

我有两张这样的桌子

    WITH table1 AS (
  SELECT TO_DATE('03/24/2015 11:30:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col FROM DUAL UNION ALL
  SELECT TO_DATE('08/03/2016 07:15:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col FROM DUAL UNION ALL
  SELECT TO_DATE('02/29/2016 22:30:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col FROM DUAL
),
table2 AS (
  SELECT TO_DATE('03/20/2015 11:30:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col, 'table2_row1' AS data_col FROM DUAL UNION ALL
  SELECT TO_DATE('03/21/2015 11:30:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col, 'table2_row1' AS data_col FROM DUAL UNION ALL
  SELECT TO_DATE('03/25/2015 11:30:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col, 'table2_row2' AS data_col FROM DUAL UNION ALL
  SELECT TO_DATE('08/02/2016 07:15:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col, 'table2_row3' AS data_col FROM DUAL UNION ALL
  SELECT TO_DATE('08/04/2016 07:15:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col, 'table2_row4' AS data_col FROM DUAL UNION ALL
  SELECT TO_DATE('02/28/2016 22:30:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col, 'table2_row5' AS data_col FROM DUAL UNION ALL
  SELECT TO_DATE('03/01/2016 22:30:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col, 'table2_row6' AS data_col FROM DUAL
)

select * from table1

如何找到table2中较小的最近日期？

Answer 1

我们可以用简单易懂的方法：

WITH table1 AS (
  SELECT TO_DATE('03/24/2015 11:30:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col FROM DUAL UNION ALL
  SELECT TO_DATE('08/03/2016 07:15:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col FROM DUAL UNION ALL
  SELECT TO_DATE('02/29/2016 22:30:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col FROM DUAL
),
table2 AS (
  SELECT TO_DATE('03/20/2015 11:30:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col, 'table2_row1' AS data_col FROM DUAL UNION ALL
  SELECT TO_DATE('03/21/2015 11:30:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col, 'table2_row1' AS data_col FROM DUAL UNION ALL
  SELECT TO_DATE('03/25/2015 11:30:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col, 'table2_row2' AS data_col FROM DUAL UNION ALL
  SELECT TO_DATE('08/02/2016 07:15:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col, 'table2_row3' AS data_col FROM DUAL UNION ALL
  SELECT TO_DATE('08/04/2016 07:15:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col, 'table2_row4' AS data_col FROM DUAL UNION ALL
  SELECT TO_DATE('02/28/2016 22:30:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col, 'table2_row5' AS data_col FROM DUAL UNION ALL
  SELECT TO_DATE('03/01/2016 22:30:00', 'mm/dd/yyyy hh24:mi:ss') AS date_col, 'table2_row6' AS data_col FROM DUAL
)

SELECT *
FROM (
  SELECT table1.date_col,
    table2.date_col AS nearest_smaller_date,
    table2.data_col,
    ROW_NUMBER() OVER (PARTITION BY table1.date_col ORDER BY ABS(table1.date_col - table2.date_col)) AS rn
  FROM table1
  JOIN table2 ON table1.date_col >= table2.date_col
)
WHERE rn = 1
;