Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

From: VC <boston103_at_hotmail.com>
Date: Sun, 3 Jul 2005 22:24:23 -0400
Message-ID: <Kp-dnXo1K5JYAFXfRVn-2w@comcast.com>

"Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message news:SiSxe.136301$no5.7264319_at_phobos.telenet-ops.be...
> VC wrote:

>> "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message 
>> news:I4Cxe.135796$Bh7.7066690_at_phobos.telenet-ops.be...
>>
>>>Jon Heggland wrote:
>>
>> [...]
>>
>>>>Not personally, but what more do you need than definitions of value, 
>>>>domain, tuple and relation, and a minimal set of algebra operators?
>>>
>>>The notions of database schema, database constraints, database instances 
>>>and how they are exactly related.
>>  A constraint (in the RM) is just a predicate [required to evaluate to 
>> true].
>

> A predicate over what? Before you can define a predicate you have to
> define the domains it applies to.

With the RM, sort of obvious, no ?

Let's assume we have a relational variable :

var PERSON relation {ssn# in ssn_type, name in name_type, age in integer, salary in integer} key{ssn#};

We can define a predicate (constraint): forAll ssn#, name, age,salary: {ssn#, name, age,salary} in PERSON & (age < 14) -> salary = 0

that says that everyone described by the PERSON relational variable (which may be a view btw) and younger than 14 earns 0. This would be an example of a relation constraint (applicable to one relational variable only).

In a similar way, one can define predicates involving two or more relational variables. The resulting constraints are usually called database constraints.

Additionally, the relational variable definition itself includes attribute constraints (name in name_type, age in integer and so on).

>
>> A relation schema is a schema name R and a set of attributes A: R(A)
>

> You forgot to model the domains. The attributes have to be associated with
> a domain.

That is correct. A domain, or a data type, is pair (V, O) where V is a set of values and O is a set of operations closed over V [I believe Codd did not require O so that (V, O) can be collapsed to just V. 'O' is an attempt to accomodate user defined data types (or 'objects') in the RM].

>
>> A database schema is a pair (RR, C) where RR is a set of relation 
>> schemata and C is a set of constraints on RR.
>

> You forgot to model that relations must have a unique name. And what is
> exaclty a "set of constraints on RR"? You didn't define that properly.

I did not. See above: "a relation schema is a schema *name* ...". C is a collection of database constraints, relation constraints, attribute constraints and, if one admits user defined types or 'objects', type constraints. Simple, no ?, especially in comparison to alternative data models.

>
>> A relation instance for R(A) is a set of tuples.
>

> You forgot to model that the tuples have to be consistent with the header
> of the relation.

I did not, see the R(A) notation.

>
> -- Jan Hidders
Received on Sun Jul 03 2005 - 21:24:23 CDT