Re: three-way join - solved - Mailing list 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
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