Re: Expressing SQL in relational algebra

> One does not express axioms or postulates using an algebra. One uses
> an algebra to prove derivable facts from the axioms. One would express
> "John likes Mary" using a tuple. One would use the algebra to derive
> the tuples that satisfy some predicate such as "Who loves Mary?"

I was under the impression relational algebra could be used to find things in an ideal database. Is this true or false?

If true, I was wondering what is the relational algebra expression to find "John likes Mary" and not other combinations such as "Mary likes John", "like John Mary", etc.

By relational expression, I meant using basic operations such as intersection, union, negation, and grouping ("()"). I do not consider "SELECT", "FROM", "WHERE" to be basic relational operators.

I am experimenting with relational expressions to find data rather than SQL-like statements but having trouble with "John likes Mary" because the expression does not constrain the order. See bottom of www.xdb1.com/Detail.asp for a sample algebra expression to find a person where the order within a set of () is not important. Received on Mon Apr 07 2003 - 21:40:51 CEST