Skip site navigation (1) Skip section navigation (2)

Re: three-way join - solved

From: Gary Stainburn <gary(dot)stainburn(at)ringways(dot)co(dot)uk>
To: <pgsql-sql(at)postgresql(dot)org>
Subject: Re: three-way join - solved
Date: 2004-04-19 16:13:22
Message-ID: 200404191713.22957.gary.stainburn@ringways.co.uk (view raw or flat)
Thread:
Lists: pgsql-sql
On Monday 19 April 2004 4:01 pm, Gary Stainburn wrote:
> On Monday 19 April 2004 3:06 pm, Stijn Vanroye wrote:
> > Gary wrote:
> > > Hi folks,
> > >
> > > here's a straight forward join that I simply can't get my head round.
> > >
> > > I've got
> > >
> > > consumables:	cs_id, cs_make, cs_comments
> > > cons_locations: cl_id, cl_desc
> > > cons_stock: cs_id, cl_id, status (1=ordered, 2=in-stock)
> > >
> > > (one stock record per stock item, qty=3 means 3 records)
> >
> > assuming that the PK's are:
> > consumables : cs_id
> > cons_loacations: cl_id
> > cons_stock: cs_id, cl_id
> > You could only have 1 record in cons_stock for each unique combination of
> > consumable and location. If the primary key for cons_stock would also
> > include the field status you could have 2 records for each unique
> > combination of  consumable and location, one where status is ordered, and
> > one where status is in-stock.
>
> Sorry for the confusion.  For the purpose of simplicity I trimmed the info
> - a little too far it seems.
>
> cons_stock has as it's PK a serial field, cost_id (cost_ is the prefix I
> use for fields on this table, the other fields therefore are cost_cs_id and
> cost_cl_id).  I need to be able to track individual items, and thus give it
> a unique id.  If I ordered 4 HP 4100 toners, they'd create 4 seperate
> records even though the cost_cs_id and cost_cl_id's would all be the same.
>
> > > I'm struggling to create a quiery to produce
> > >
> > > cs_id, cs_make, cs_comments, cl_desc, hand_qty, order_qty
> > >
> > > where hand_qty and order_qty is the number of records grouped
> > > by cs_id, cl_id,
> > > and status.
> >
> > Given the previous, the result for qty would be pretty obvious I think,
> > since you would have only 1 record for the combination cs_id,cl_id and
> > status.
> >
> > > I've done the simple part and created a view balances to
> > > tally the cons_stock
> > > as:
> > >
> > > create view balances as
> > >   select cost_cs_id, cost_cl_id, cost_css_id, count(*) as qty
> > >     from cons_stock
> > >     group by cost_cs_id, cost_cl_id, cost_css_id;
> >
> > I don't understand where the cost_* fields come from, especially the
> > cost_css_id field. Assuming that these fields are the cs_id, cl_id and
> > status qty is most likley going to be 1 all the time? Maybe it's worth to
> > rethink your database structure, or adding the qty fields to the table
> > cons_stock and keeping them up-to-date? (eg. CONS_STOCK (cs_id, cl_id,
> > hand_qty, order_qty) PK(cs_id, cl_id) ) that way you simply change the
> > quantity fields for each combination of location-consumable according to
> > the situation (and sound the alarm if the reach a certain level?).
>
> the cost_ (abrev of cons_stock) is the prefix of the fields on the
> cons_stock field. consumables have prefix cs_ and locations have cl_.
> Therefore when cons_stock references consumables id field it is called
> cost_cd_id.
>
> > If anyone thinks I'm wrong, please correct me.
>
> I hope my my explanation's cleared up the grey area.  I've included all of
> the relevent schema below to help show what I want.
>
> create table cons_types (
>   cst_id		serial not null unique,
>   cst_desc	varchar(40),
>   primary key (cst_id)
> );
> insert into cons_types (cst_desc) values ('Toner cartridge');	-- 1
> insert into cons_types (cst_desc) values ('Ink cartridge');	-- 2;
>
> create table consumables (
>   cs_id		serial not null unique,
>   cs_make	varchar(40),
>   cs_code	varchar(20),
>   cs_type	int4 references cons_types(cst_id) not null,
>   cs_colour	varchar(40),
>   cs_comments	text,
>   primary key (cs_id)
> );
>
> insert into consumables (cs_make, cs_code, cs_type,cs_colour, cs_supp,
> cs_comments) values
> 	('HP', 'C4096A', 1, 'BLACK', 5, '2100	2	0');
>
> create table cons_locations (
>   cl_id		serial not null unique,
>   cl_desc	varchar(40),
>   primary key (cl_id)
> );
> insert into cons_locations (cl_desc) values ('Leeds Computer Room');	-- 1
>
> create table cons_status (
>   css_id		serial not null unique,
>   css_desc	varchar(40),
>   primary key (css_id)
> );
> insert into cons_status (css_desc) values ('Ordered');	-- 1
> insert into cons_status (css_desc) values ('In Stock');	-- 2
> insert into cons_status (css_desc) values ('Issued');	-- 3
>
> create table cons_stock (
>   cost_id	serial not null unique,
>   cost_cs_id	int4 references consumables(cs_id) not null,
>   cost_css_id	int4 references cons_status(css_id) not null,
>   cost_cl_id	int4 references cons_locations(cl_id) not null,
>   cost_supp	int4 references contacts(co_id),
>   cost_comments	text,
>   primary key (cost_id)
> );
> -- insert 2 HP 2100 toners in stock at Leeds
> insert into cons_stock (cost_cs_id, cost_css_id, cost_cl_id) values
> 	(1, 2, 1);
> insert into cons_stock (cost_cs_id, cost_css_id, cost_cl_id) values
> 	(1, 2, 1);
>
> create view balances as
>   select cost_cs_id, cost_cl_id, cost_css_id, count(*) as qty
>     from cons_stock
>     group by cost_cs_id, cost_cl_id, cost_css_id;
>
> > Regards,
> >
> > Stijn Vanroye

I've sorted it.  

Firstly, I've done away with the status field.  If an item's been issued or is 
still on order it does not have a location.  I've therefore set up two 
locations, one of 'On Order' and one of 'Issued'. That's got rid of one 
table/relationship.

The relationship between the stock and the locations is simple. Each stock 
item has a location.  I therefore do a straight forward join to end up with 
stock+location information.

The Stock->Consumable relation is just as simple.  I do a straight forward 
join of the new stock+location data with the consumables data, and end up 
with what I need, consumable, location and quantity details, i.e. 

create view stock as
   select c.*, b.cost_cl_id, b.cl_desc, b.qty from consumables c, 
      (select b.*, cl.cl_desc 
          from balances b, cons_locations cl 
          where b.cost_cl_id = cl.cl_id 
          order by cost_cs_id) b
       where c.cs_id = b.cost_cs_id;

-- 
Gary Stainburn
 
This email does not contain private or confidential material as it
may be snooped on by interested government parties for unknown
and undisclosed purposes - Regulation of Investigatory Powers Act, 2000     


In response to

pgsql-sql by date

Next:From: Bruno Wolff IIIDate: 2004-04-19 17:45:51
Subject: Re: three-way join
Previous:From: James RobinsonDate: 2004-04-19 15:56:06
Subject: Prepared Statements and large where-id-in constant blocks?

Privacy Policy | About PostgreSQL
Copyright © 1996-2014 The PostgreSQL Global Development Group