如何從Teradata中的表中選擇下一個日期

Question

我有包含以下信息的表

Quarter_END_DATE | Flag 
2014/31/03       |  N 
2014/30/06       |  Y 
2014/30/09       |  N 
2014/31/12       |  N 

等等....

每當運行查詢時，都應將下一個四分之一flag to Y更新flag to Y ，將previous to N四分之一flag to Y更新previous to N

請提供實施此解決方案。

Answer 1

將下一個季度的標志更新為Y （假設表中只有一個Y標志）：

UPDATE [table_name] SET Flag = 'Y'
WHERE Quarter_END_DATE = DATEADD(day, 90, (
   SELECT FIRST(Quarter_END_DATE)
   FROM [table_name]
   WHERE Flag = 'Y'));

將上一季度的標志更新為N ：

UPDATE [table_name] SET Flag = 'N'
WHERE Quarter_END_DATE = (
   SELECT MIN(Quarter_END_DATE)
   FROM [table_name]
   WHERE Flag = 'Y'));

Answer 2

UPDATE tab
FROM 
 ( SELECT Quarter_END_DATE, Flag
   FROM tab
   QUALIFY Flag = 'Y'   -- current row with 'Y'
        OR MAX(Flag)    -- next row to be set to 'Y'
           OVER (ORDER BY Quarter_END_DATE
                 ROWS BETWEEN 1 PRECEDING AND  1 PRECEDING) = 'Y'
 ) AS src
SET Flag = CASE WHEN src.Flag = 'Y' THEN 'N' ELSE 'Y' END -- switch the flag
WHERE tab.Quarter_END_DATE = src.Quarter_END_DATE
  AND tab.Flag = src.Flag
;

Answer 3

最好是指出要使用的是什么dbms，但是，如果在mysql中執行此操作，則可以：

update [table_name] set flag = 'N'
where flag = 'Y'
;

update [table_name] set flag = 'Y'
where quarter_end_date between now() and dateadd(now(), 90 DAYS);