Re: Codd's Information Principle

From: Bob Badour <>
Date: Fri, 30 Oct 2009 19:33:01 -0300
Message-ID: <4aeb699d$0$5355$>

paul c wrote:

> wrote:

>> On Oct 30, 11:41 am, paul c <> wrote:
>>> Yes, I mean assignment to a (programming) variable. I don't know of any
>>> logical operator called 'assignment'.
>> You don't seem understand assignment.
>> Assignment has nothing to do with the relationship between relations
>> and logic or the relational model per se. The semantics of assignment
>> in a dbms language is exactly the same as it is in every programming
>> language.
>> You evaluate expressions using the current values of variables. You
>> can assign the value of an expression to a variable. This gives a new
>> overall state. The relationship between the values of one or more
>> variables in the post-(multiple-)assignment state and the expressions
>> that were used to express those values in the pre-assignment state
>> is simply (and tautologically) that the values of the assigned
>> expressions evaluated using the pre-assignment variable values are
>> the same as the values of the expressions consisting of just the
>> variable names evaluated using the post-assignment variable values
>> (ie the post-assignment values of the variables).
>> It doesn't matter what the types of variables are.
>> If a language allows assignment to an expression then the language
>> designer has to say what that means. Presumably the
>> post-assignment values of the variables in the assigned-to expressions
>> must be such that when they are evaluated in the post-assignment
>> state then their values are the same as the values of the assigned-
>> from
>> expressions evaluated in the pre-assignment state. If the language
>> processor cannot determine a single choice for the post-assignment
>> values of variables then the language designer has to give some sort
>> of
>> policy. But again, the particular assigned-from expressions have no
>> other relationship with the new state than that when evaluated in the
>> old state they give the values of the variables in the new state. The
>> particular assigned-from expressions don't matter, just their values.
>> philip
> That's adding more notions, such as 'state' and 'before' and 'after', 
> 'assignment to an expression' et al, all artifacts of certain physical 
> programming attitudes if you ask me.  I'm more interested in subtracting 
> notions as far as possible.

To be fair to philip, those are exactly the notions involved with imperative programming languages. No need to invent anything else to describe them. View updates are assignments to expressions. Received on Fri Oct 30 2009 - 23:33:01 CET

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