Re: Lowest 2 items per - Mailing list pgsql-sql

On Jun 1, 2012, at 10:34, "Relyea, Mike" <Mike.Relyea@xerox.com> wrote:

> I need a little help putting together a query.  I have the tables listed
> below and I need to return the lowest two consumables (ranked by cost
> divided by yield) per printer, per color of consumable, per type of
> consumable.
>
> CREATE TABLE printers
> (
>  printerid serial NOT NULL,
>  make text NOT NULL,
>  model text NOT NULL,
>  CONSTRAINT printers_pkey PRIMARY KEY (make , model ),
>  CONSTRAINT printers_printerid_key UNIQUE (printerid ),
> )
>
> CREATE TABLE consumables
> (
>  consumableid serial NOT NULL,
>  brand text NOT NULL,
>  partnumber text NOT NULL,
>  color text NOT NULL,
>  type text NOT NULL,
>  yield integer,
>  cost double precision,
>  CONSTRAINT consumables_pkey PRIMARY KEY (brand , partnumber ),
>  CONSTRAINT consumables_consumableid_key UNIQUE (consumableid )
> )
>
> CREATE TABLE printersandconsumables
> (
>  printerid integer NOT NULL,
>  consumableid integer NOT NULL,
>  CONSTRAINT printersandconsumables_pkey PRIMARY KEY (printerid ,
> consumableid ),
>  CONSTRAINT printersandconsumables_consumableid_fkey FOREIGN KEY
> (consumableid)
>      REFERENCES consumables (consumableid) MATCH SIMPLE
>      ON UPDATE CASCADE ON DELETE CASCADE,
>  CONSTRAINT printersandconsumables_printerid_fkey FOREIGN KEY
> (printerid)
>      REFERENCES printers (printerid) MATCH SIMPLE
>      ON UPDATE CASCADE ON DELETE CASCADE
> )
>
> I've pulled together this query which gives me the lowest consumable per
> printer per color per type, but I need the lowest two not just the first
> lowest.
>
> SELECT printers.make, printers.model, consumables.color,
> consumables.type, min(cost/yield) AS cpp
> FROM printers
> JOIN printersandconsumables ON printers.printerid =
> printersandconsumables.printerid
> JOIN consumables ON consumables.consumableid =
> printersandconsumables.consumableid
> WHERE consumables.cost Is Not Null
> AND consumables.yield Is Not Null
> GROUP BY printers.make, printers.model, consumables.color,
> consumables.type
> ORDER BY make, model;
>
>
> After doing a google search I didn't come up with anything that I was
> able to use so I'm asking you fine folks!
>
> Mike
>
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I would recommend using the "RANK" window function with an appropriate partition clause in a sub-query then in the
outerquery you simply WHERE rank <= 2 

You will need to decide how to deal with ties.

David J.