Dynamic start date from specific column in a table (sysdate)

Question

I am pretty new in this field, trying to learn slowly so please be patient with me:)

My database contains a table called t_usage_interval. In this table there is a column name ID_Interval. Each month a new random 10 digit number is created in this column.

This is the query I am using

I would like to find out if there is a way to pull the latest interval by using column name DT_START with SYSDATE option? I guess it would be a dynamic query search from a sysdate to display the latest ID_Interval?

Thank you, A

Answer 1

This is how I understood the question.

A straightforward query returns row(s) whose dt_start is the first in that table that is lower or equal to sysdate (you might also use trunc(sysdate) , if you don't care about time component). Drawback of this query is that it scans t_usage_Interval table twice.

select *
from t_usage_interval a
where a.dt_start = (select max(b.dt_start)
                    from t_usage_interval b
                    where b.dt_start <= sysdate
                   );
                   

Somewhat less intuitive option is to "rank" rows (whose dt_start is lower than sysdate) by dt_start , and then return row(s) that rank the "highest". This option scans the table only once, so it should perform better.

with temp as
  (select a.*,
     rank() over (order by a.dt_start desc) rn
   from t_usage_interval a
   where a.dt_start <= sysdate
  )
select t.*
from temp t
where t.rn = 1;