Cannot see any problem with subquery i.e. why the second query is bad? BTW both statistics are copied from the second run of each select to avoid first time parsing overhead. Anyway to get the maximum count(*) for each city you need to scan all table or probably all index if the column is not null.
SQL> set arraysize 100 SQL> set autotrace on SQL> create table temp as select * from all_objects;
Table created.
Elapsed: 00:00:03.05 SQL> select count(*) cnt, owner 2 from temp group by owner 3 order by cnt desc;
CNT OWNER ---------- ------------------------------ 13893 SYS 12287 PUBLIC ... 51 rows selected.
Elapsed: 00:00:00.03 Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 805 consistent gets 0 physical reads 0 redo size 2350 bytes sent via SQL*Net to client 495 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 2 sorts (memory) 0 sorts (disk) 51 rows processed SQL> ed Wrote file afiedt.buf
1 select * from ( 2 select count(*) cnt, owner 3 from temp group by owner 4 order by cnt desc 5 ) 6* where rownum <=1 SQL> /
CNT OWNER ---------- ------------------------------ 13893 SYS
Elapsed: 00:00:00.00 Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 805 consistent gets 0 physical reads 0 redo size 388 bytes sent via SQL*Net to client 495 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 2 sorts (memory) 0 sorts (disk) 1 rows processed
SQL>
Hi Muhammed
It's just a case study. Is it possible to do without the sub query?
Actually, the original question is not from me, but from a friend of mine. I was just thinking about a good solution all morning long and couldn't figure out a good answer to the question.
I find it quite reasonable to try a simple thing like a Top-N analysis without the use of a (possibly costly) sub query.
But maybe I'm just spoiled by the proprietary MySQL feature of LIMIT BY…
Regards,
Tristan
------------------------------------------------------------------------------------
Tristan van Essen
Accenture Technology Solutions AG
Baslerstrasse 60
CH-8048 Zürich
Office: +41 44 405 3245
Mobile: +41 79 654 0592
Email: tristan.van.essen@xxxxxxxxxxxxx ________________________________
From: Muhammed Soyer [mailto:msoyer@xxxxxxxxx] Sent: Mittwoch, 7. Juni 2006 12:02 To: van Essen, Tristan Cc: oracle-l@xxxxxxxxxxxxx Subject: Re: How to do a Top-N analysis, while avoiding subqueries
Why dont you want to use sub queries ? Any solid reason or just for curiosity ..
2006/6/7, tristan.van.essen@xxxxxxxxxxxxx <tristan.van.essen@xxxxxxxxxxxxx>:
Hi folks, this is my first question in this mailing list.
How do you do a top-n analysis while avoiding subqueries, but still use group functions on other columns?
I have following Query:
SELECT COUNT(city) quantity, city
FROM customer
GROUP BY city
ORDER BY quantity DESC;
This works fine so far with the exception that I only want the first record returned, instead of all. I tried to solve the problem with the ROWNUM function, but it won't work well. The problem here is that I must include ROWNUM in the GROUP BY clause, else the query fails. But then the result isn't right anymore. I wish there was any sort of LIMIT BY clause, like there is in MySQL.
How can I solve this, while not using any subqueries?
Regards,
Tristan
------------------------------------------------------------------------------------
Tristan van Essen
Accenture Technology Solutions AG
Baslerstrasse 60
CH-8048 Zürich
Office: +41 44 405 3245
Mobile : +41 79 654 0592
Email: tristan.van.essen@xxxxxxxxxxxxx
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