Обсуждение: select-querries out of an array?

Hiho,

I have a question which neither the samples (as far as I was able to
figure out) or the documentation were able to clear up.

I have a table definition like

(btw. why isn't the primary key of the Spiel-class inherited by the
 children classes???)

create table Spiel (
 Name                text not null,
 Bestellnummer          text not null primary key,
 HerstellerID            int4 not null,
 Erscheinungsdatum        datetime,
 Preisempfehlung         money not null,
 Beschreibung            text[],
 Computertyp            text[] not null,
 Datentraegertyp        text[] not null,
 ClueBook            text[] );

create table rollenspiel (
 Bestellnummer          text not null primary key,
 System                text not null,
 AnzahlCharaktere        int2 not null,
 Typ                text not null,
 SpielstandEditor        text[] ) inherits ( spiel );

create table adventure (
 Bestellnummer          text not null primary key,
 Schwierigkeitsgrad        int2 not null ) inherits ( spiel );

create table simulation (
 Bestellnummer          text not null primary key,
 Typ                text[] not null,
 AnzahlChars            int2 not null ) inherits ( spiel );

create table action (
 Bestellnummer          text not null primary key,
 Eingabegeraet            text[] not null,
 AnzahlSpieler            int2 not null ) inherits ( spiel );

with several subclasses which inherit attributes from the parent class.

When I now would like to have all games (Spiel*) selected, which come
on CD as media (Datentraegertyp is array which contains CD, Disks, misc,
etc.), I would type the sql request like this:

select name from Spiel* where Datentraegertyp = '{"CD"}';

which only lists all games which *ONLY* come on CD (pretty clear as the
match is '=' on the single array entry. How do I formulate the request when
I would like to have those which have Datentraegertyp like '{"CD", "Diskette"}'
or '{"Tape", "Diskette", "CD"}' ?

I hope you get what I am up to ;-)

Thanks once again for hints.

At 17:03 +0200 on 6/10/98, Gemeinschaft Studienarbeit Datenbanken wrote:


> When I now would like to have all games (Spiel*) selected, which come
> on CD as media (Datentraegertyp is array which contains CD, Disks, misc,
> etc.), I would type the sql request like this:
>
> select name from Spiel* where Datentraegertyp = '{"CD"}';
>
> which only lists all games which *ONLY* come on CD (pretty clear as the
> match is '=' on the single array entry. How do I formulate the request when
> I would like to have those which have Datentraegertyp like '{"CD",
>"Diskette"}'
> or '{"Tape", "Diskette", "CD"}' ?
>
> I hope you get what I am up to ;-)
>
> Thanks once again for hints.

I never liked arrays in databases. They are not the right implementation,
at least not from the classical relational point-of-view, for what you have
in mind.

I tend to look at arrays as representing a bulk of information (An icon, an
IP). And that's exactly what you can do with it in PostgreSQL: Retrieve the
entire array and use it within the application, write an entire array, etc.

The relational approach to your configuration is to have a second table,
relating to your main table through the primary key. So, it has two fields
- "Bestellnummer" (whatever that is) and "Datentraegertyp". For one game in
the Spiel* tables, you can have several rows in this secondary table - one
row for each media on which this particular game appears.

Then you join.

SELECT ....
FROM Spiel* s, MediaTable m
WHERE .....
  AND s.Bestellnummer = m.Bestellnummer
  AND m.Datentraegertyp in ( 'CD', 'Diskette', 'Tape' );

I haven't tried this with an object-oriented table (with that * after the
table name).

Herouth

--
Herouth Maoz, Internet developer.
Open University of Israel - Telem project
http://telem.openu.ac.il/~herutma