how to difference of row numbers approach

问题

i have data like this table

ItemId  Value   Date
1        2      2017-12-18 17:00:00.000
1        2      2017-12-18 17:02:00.000
1        2      2017-12-18 17:04:00.000
1        3      2017-12-18 17:06:00.000
1        3      2017-12-18 17:08:00.000
1        2      2017-12-18 17:10:00.000
1        2      2017-12-18 17:12:00.000
1        2      2017-12-18 17:16:00.000
1        4      2017-12-18 17:14:00.000

i want to output like this in sql server

ItemId     Value   MaxDate
1          2       2017-12-18 17:04:00.000
1          3       2017-12-18 17:08:00.000
1          2       2017-12-18 17:16:00.000
1          4       2017-12-18 17:14:00.000

thanks for your anwsers.

回答1:

You appear to want the last row before value changes, although I'm not sure where value "4" comes from (my best guess is that the last input row should have a "4" and a different timestamp).

If so, you can simply use lead():

select t.*
from (select t.*,
             lead(value) over (partition by itemId order by date) as next_value
      from t
     ) t
where next_value is null or next_value <> value;

来源：https://stackoverflow.com/questions/47887679/how-to-difference-of-row-numbers-approach