SQL: retrieve only the records whose value has changed

Question

Sorry for the nondescript title. I\'ll edit as we go along.

I have a table RateTable:

| Code   |  Date     |   Rate  |

  B001     2009-         


        

Answer 1

If your RDBMS supports analytic functions then the optimum method is almost certainly this:

select code, date, rate, last_rate
from
(
select code,
       date,
       rate,
       lag(rate) over (partition by code order by date) last_rate
from   ratetable
) my_tb
where  my_tb.rate != my_tb.last_rate