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Re: Critique needed for contact-DB draft

From: Steve Crawford <scrawford(at)pinpointresearch(dot)com>
To: "Felix E(dot) Klee" <felix(dot)klee(at)inka(dot)de>, pgsql-novice(at)postgresql(dot)org
Subject: Re: Critique needed for contact-DB draft
Date: 2004-07-15 05:30:26
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Lists: pgsql-novice
On Wednesday 14 July 2004 5:27 pm, Felix E. Klee wrote:
> ...I bluntly ask you to criticize the design attached below...
> I'm especially interested on your take of my use of arrays. They
> avoid the need for additional tables, but maybe they are not good,
> nevertheless?

Correct, they are not. Failure to create a well thought out database 
structure will cause you to lose the ability to realize the power of 
a SQL database and will be much harder to correct later than earlier. 
I know you are under "temporal pressure" but this is truely a case of 
"pay me now or pay me later".

By way of example, say you want to write a query showing all contacts 
for a particular location. With an array of 0..n location elements 
that is very hard to do.

Additionally, basic integrity checks that are easy to establish at the 
database level if the data structure is sound will be difficult to 
impossible with this array structure. An example here is ensuring 
that a location appears only once for a given contact and that all 
locations for a contact exist in the location table (and further, 
that the location record cannot be deleted if it is referenced by any 
other table).

> BTW, what I'm really missing is as a newbie is a way to define new
> data types as "structures" as known from e.g. the C programming
> language (or is this supported by PostgreSQL?). That would IMHO be
> a more natural concept for storing data (for example, then I could
> create an array of addresses for each contact).

No, what you are actually missing is experience "thinking in SQL." You 
are trying to develop in SQL while thinking in C. This will yield 
less than satisfactory results.

Generally you should ask of each piece of data is it multivariate (an 
address field instead of separate fields for streetaddress, city, 
state and zip for example) and is it multivalued (ie. a field with 
0..n phone numbers). If either of these exist then there is almost 
certainly something wrong with the design as this is basically the 
first step in data normalization.

You may want to check out "Database Design for Mere Mortals".

SQL has its roots in set theory. I can't try to write your entire 
schema but will offer an example to get you started. Say you have 
some people (name, sex, birthdate) and those people have zero or more 
phone numbers. You also have some organizations (name, president) to 
which those people belong. You might consider the following:

person_id integer (primary key==> not null, unique - probably use a 
sequence number)
name text,
sex char(1) (validated as M or F)
birthdate date (validated as appropriate for your data)

phone_id integer (primary key ==> not null, unique - use a sequence)
phone number (validate as appropriate or even split into ac, prefix, 
number if necessary for your app_
number type (home, work, fax, cell, etc. - this can be hard coded but 
could also be validated against a "phone types" table)

org_id integer (primary key ==> not null, unique, use a sequence)
president integer (links to people database)

people_organizations (a "linking table")
person_id integer
org_id integer

people_phones (another "linking table")
person_id integer
phone_id integer

The linking table is the key to your array problem. It allows you to 
set up all sorts of many-to-many relationships and still enforce data 
integrity and perform efficient queries.

A person can belong to several organizations and organizations will 
presumably have many members but organizing the data this way allows 
you to easily answer questions like how many organizations does Bob 
belong to, how many members does Fooorg have, who belongs to more 
than two organizations, etc.

Organize your data right and you can run all sorts of queries easily. 
Do it wrong and you will suffer.

Hope this helps.


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Subject: Re: Critique needed for contact-DB draft
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