Re: Extracting data where a column is max - Mailing list pgsql-novice

Hi Michael,

I ended up with this query as I do not really care about the inventory_id in
the end game.  What about SELECT DISTINCT is non-standard?  Are there any
implications of using this other than portability?

SELECT DISTINCT ON ( inventory.tbl_data.item_id )
       inventory.tbl_data.item_id,
       inventory.tbl_data.quantity
  FROM inventory.tbl_data
 ORDER BY inventory.tbl_data.item_id,
          inventory.tbl_data.inventory_id DESC;

Kind Regards,
Keith

> On Thu, Dec 23, 2004 at 03:57:46PM -0500, Keith Worthington wrote:
>
> > I have the following data:
> > IPADB=# SELECT * FROM inventory.tbl_data;
> >  inventory_id |  item_id   | quantity
> > --------------+------------+----------
> >             1 | RMFPB14BK  |      551
> >             1 | RPP3S114BK |      629
> >             1 | RPP3S14YL  |     1009
> >             1 | TW360PYSD  |      444
> >             1 | TW360PYWH  |      910
> >             6 | 004173-1   |       44
> >             6 | RMFPB14BK  |      399
> >             6 | RPP3S14YL  |     1233
> >             9 | RPP3S14YL  |       50
> > (9 rows)
> >
> > I want to retrieve the item_id and the quantity corresponding to the maximum
> > inventory_id.  I can get the proper item_id.
>
> If you don't mind using a non-standard construct then you could use
> SELECT DISTINCT ON.  For more info see the "SELECT" and "Select
> Lists" documentation.
>
> SELECT DISTINCT ON (item_id) *
> FROM tbl_data
> ORDER BY item_id, inventory_id DESC;
>
>  inventory_id |  item_id   | quantity
> --------------+------------+----------
>             6 | 004173-1   |       44
>             6 | RMFPB14BK  |      399
>             1 | RPP3S114BK |      629
>             9 | RPP3S14YL  |       50
>             1 | TW360PYSD  |      444
>             1 | TW360PYWH  |      910
> (6 rows)
>
> The ORDER BY specification is important.  If you need a different
> order in the final result then you can use a sub-select:
>
> SELECT * FROM (
>   SELECT DISTINCT ON (item_id) *
>   FROM tbl_data
>   ORDER BY item_id, inventory_id DESC
> ) AS s
> ORDER BY inventory_id, item_id;
>
>  inventory_id |  item_id   | quantity
> --------------+------------+----------
>             1 | RPP3S114BK |      629
>             1 | TW360PYSD  |      444
>             1 | TW360PYWH  |      910
>             6 | 004173-1   |       44
>             6 | RMFPB14BK  |      399
>             9 | RPP3S14YL  |       50
> (6 rows)
>
> > BTW The SQL code to create the table and data is below.  (Are ya proud of me
> > Michael? ;-) )
>
> :-)
>
> --
> Michael Fuhr
> http://www.fuhr.org/~mfuhr/


Kind Regards,
Keith Worthington
President

Narrow Path, Inc.
520 Trumbull Highway
Lebanon, CT 06249-1424
Telephone:  (860) 642-7114
Facsimile:  (860) 642-7290
Mobile:     (860) 608-6101

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