Re: Codd's Information Principle

From: <>
Date: Fri, 30 Oct 2009 12:46:20 -0700 (PDT)
Message-ID: <>

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 assignedfrom

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 Received on Fri Oct 30 2009 - 20:46:20 CET

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