How do I limit the number of rows returned by an Oracle query after ordering?

Question

Is there a way to make an Oracle query behave like it contains a MySQL limit clause?

In MySQL, I can do this:

Answer 1

Pagination queries with ordering are really tricky in Oracle.

Oracle provides a ROWNUM pseudocolumn that returns a number indicating the order in which the database selects the row from a table or set of joined views.

ROWNUM is a pseudocolumn that gets many people into trouble. A ROWNUM value is not permanently assigned to a row (this is a common misunderstanding). It may be confusing when a ROWNUM value is actually assigned. A ROWNUM value is assigned to a row after it passes filter predicates of the query but before query aggregation or sorting.

What is more, a ROWNUM value is incremented only after it is assigned.

This is why the followin query returns no rows:

 select * 
 from (select *
       from some_table
       order by some_column)
 where ROWNUM <= 4 and ROWNUM > 1; 

The first row of the query result does not pass ROWNUM > 1 predicate, so ROWNUM does not increment to 2. For this reason, no ROWNUM value gets greater than 1, consequently, the query returns no rows.

Correctly defined query should look like this:

select *
from (select *, ROWNUM rnum
      from (select *
            from skijump_results
            order by points)
      where ROWNUM <= 4)
where rnum > 1; 

Find out more about pagination queries in my articles on Vertabelo blog:

• Oracle ROWNUM Explained
• Top-N and pagination queries