Re: In an RDBMS, what does "Data" mean?
Date: Tue, 8 Jun 2004 11:08:29 -0400
Message-ID: <Yuqdneo3DMOLS1jdRVn-hw_at_comcast.com>
"Paul" <paul_at_test.com> wrote in message news:Zqixc.12382$NK4.1717725_at_stones.force9.net...
> OK, but uniqueness constraints and referential integrity constraints are
> a very small subset of all possible constraints. They're quite simple
> for a DBMS to understand and use. What about ones that are even a little
> bit more complicated? I guess the constraints mentioned above don't
> require knowledge of particular types or operators (other than
> equality), but ones like "Age < 60" do.
I didn't mean to imply that all constraints were useful in the way I set forth. Just that some were.
The DBMS can make use of value limiting constraints to compress data better. For instance, if there is a column called
, ZIP_CODE CHAR(10) (the tenth character is for the hyphen), and a value is to be stored that is CHAR(15), but the last five characters are blanks, a suitable DBMS could go ahead and store the value anyway, knowing that it can reconstruct the CHAR(15) value later, if necessary.
The same comment goes for a "field" defined as CHAR(10) by the way.
>
> In general a constraint could be any expresson in first order logic. And
> then to complicate matters further you've got non-relational operators
> (like "<") added in.
I don't understand. What makes "<" a non relational operator? I had been
taught that
"x < y" is a relation on x and y. This was in math, not comp. sci.
Received on Tue Jun 08 2004 - 17:08:29 CEST