Question

Given a table action(start:DATE, length:NUMBER, type:NUMBER), with all records unique, I need to select (e.g.) length of last action with type Y before time X:

select action.length 
 where action.type = Y 
   and action.start is the biggest, but not greater than X

Proposed solution (improved):

  with actionView as (select * from action where type = Y and time <= X) 
select length
  from actionView
 where time = (select max(time) from actionView)

But this still envolves two selects.

What I would like to ask is it possible to perform some analytical or hierarchical or any other oracle magic to this query, to improve it?

(Probably, something like this algo is what I need, but I don’t know how to express it in SQL:

savedAction.time = MinimalTime
foreach action in actions
  if action.type = y and savedAction.time < action.time <= X
    savedAction = action
return savedAction;

)

Answer 1

Oracle has no LIMIT (PostgreSQL, MySQL) or TOP (SQL Server) clause like other RDBMS. But you can use ROWNUM for that:

SELECT *
FROM  (
    SELECT length 
    FROM   action
    WHERE  type = Y 
    AND    start < X
    ORDER  BY start DESC
    )
WHERE rownum = 1;

This way, the table will be queried once only.
The details in the manual.

---

In reply to Dems comment I quote from the link above:

If you embed the ORDER BY clause in a subquery and place the ROWNUM
condition in the top-level query, then you can force the ROWNUM
condition to be applied after the ordering of the rows.