Re: In an RDBMS, what does "Data" mean?

"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 - 10:08:29 CDT