The issue is that not only does SQL syntax force an artificial order on these clauses, but that these clauses Cannot be decomposed to be used elsewhere. I cannot reuse a WHERECLAUSE or a SELECTCLAUSE in another expression.
Some detail here. What goes on in relational algebra is that the FROMCLAUSE is encoded into an axiomatic primitive called a RELATION and you get stuff like this:
Basically every operation in relational algebra produces a primitive of type RELATION which allows for all operations to be composed like unix pipes.
My example in the previous post has a flaw where it's not clear what:
SELECTCLAUSE * WHERECLAUSE = ????
will output because there's no meaning to a SQLEXPRESSION without a FROMCLAUSE. But hopefully it illustrates the point. I'm putting this here for anyone who's nitpicky about the details. The relational algebra syntax is much more elegant.
