Re: What databases have taught me

Bob Badour wrote:
> Consider:
>
> Ra = Rb AND x = 1 AND y = 1
>
> Where Ra and Rb are two relvars. Is that still a join? To what do x and
> y refer?

There is ambiguity caused by multiple equality signs. Did you mean

(Ra = Rb) AND x = 1 AND y = 1

or

Ra := (Rb AND x = 1 AND y = 1)

The first interpretation is absurd since there is no equality operator in relational algebra. In the second case there is no longer equality, but assignment := (and I still keep the redundant brackets for clarity).

> Consider:
>
> R AND x = 1 AND y = 1
>
> where R has no x attribute. Will that cause a compile-time error? Why or
> why not?

No. The cartesian product of

R AND y = 1

with

x = 1

is the EXTEND operator.

I realize that this kind of thinking would steer quite away from exact Tutorial D semantics. BTW, only now I noticed that Tutorial D is bastardised version of D&D "A" algebra. Of course, it may be argued that the connection Tutorial D <--> "A" is better designed than SQL <--> RA, but this parallel is kind of ironic. Received on Fri Jun 23 2006 - 21:01:56 CDT