Paging with Oracle and sql server and generic paging method

Question

I want to implement paging in a gridview or in an html table which I will fill using ajax. How should I write queries to support paging? For example if pagesize is 20 and wh

Answer 1

As others have suggested, you can use rownum in Oracle. It's a little tricky though and you have to nest your query twice.

For example, to paginate the query

select first_name from some_table order by first_name

you need to nest it like this

select first_name from
  (select rownum as rn, first_name from
    (select first_name from some_table order by first_name)
  ) where rn > 100  and rn <= 200

The reason for this is that rownum is determined after the where clause and before the order by clause. To see what I mean, you can query

select rownum,first_name from some_table order by first_name

and you might get

4   Diane
2   Norm
3   Sam
1   Woody

That's because oracle evaluates the where clause (none in this case), then assigns rownums, then sorts the results by first_name. You have to nest the query so it uses the rownum assigned after the rows have been sorted.

The second nesting has to do with how rownum is treated in a where condition. Basically, if you query "where rownum > 100" then you get no results. It's a chicken and egg thing where it can't return any rows until it finds rownum > 100, but since it's not returning any rows it never increments rownum, so it never counts to 100. Ugh. The second level of nesting solves this. Note it must alias the rownum column at this point.

Lastly, your order by clause must make the query deterministic. For example, if you have John Doe and John Smith, and you order by first name only, then the two can switch places from one execution of the query to the next.

There are articles here http://www.oracle.com/technology/oramag/oracle/06-sep/o56asktom.html and here http://www.oracle.com/technology/oramag/oracle/07-jan/o17asktom.html. Now that I see how long my post is, I probably should have just posted those links...