rownum issue in oracle query

Question

I m trying to run this query but it returns zero rows. Any clues why?

Select distinct a.id
from  table1 a, table b
where  ( a.id= b.id or a.id = b.secondid ) and rownum < 200;

But if I run the above query without the ROWNUM clause it finds records:

Select distinct a.id
from  table1 a, table b
where  ( a.id= b.id or a.id = b.secondid );

I'm confused why the first query is not working.

Solution 1

Please use the thread below. Pretty good explanations. https://community.oracle.com/thread/210143 Here are two working solutions.

1. Use the row_number function for 8i and 9i :
   ROW_NUMBER() OVER (ORDER BY ASC) AS ROW_NUMBER.
2. Reuse Rownum pseudo column (This seems to be better) SELECT * FROM ( SELECT t.*, ROWNUM AS rn FROM ( SELECT * FROM mytable ORDER BY paginator, id ) t ) WHERE rn BETWEEN 1 AND 3

Solution 2

You need to apply ROWNUM after Oracle figures out which rows to return. The only reliable way is this:

SELECT * FROM (
  Select distinct a.id
  from  table1 a, table b
  where  ( a.id= b.id or a.id = b.secondid )
) WHERE ROWNUM < 200;

Comments

[Gordon Linoff]

. . saying "it is not working" is not helpful. You should say what the problem is.

[Neeraj]

not working means not getting the correct output. It returned Zero rows if i add rownum in the end.

[Neeraj]

yes thanks, i figured that out but i was thinking it should have worked before as well. like if you remove the or clause then it works.e.g. Select distinct a.id from table1 a, table b where a.id= b.id and rownum < 200;

[Ed Gibbs]

@Neeraj - there are some cases where ROWNUM will work without resorting to an outer query, but they're rare. There's a pretty good explanation on the Oracle "Ask Tom" site here.