Tom Ivar Helbekkmo wrote:
> Tom Lane <tgl@sss.pgh.pa.us> writes:
>
> >> SELECT a AS b FROM table1 ORDER BY a;
> >> I think it should be:
> >> SELECT a AS b FROM table1 ORDER BY b;
> >
> > You are correct that the latter is the SQL-standard version.
>
> In fact, this becomes pretty obvious when you consider the theoretical
> ordering of the evaluation of the various parts of the statement. The
> ORDER BY is executed *after* the SELECT part, so it only knows about
> the column name 'b', and doesn't know what 'a' refers to. Actually,
> ORDER BY isn't even part of the SELECT statement, really, but hides an
> implicit CURSOR operation taking place during data output. Allowing
> the first version to work is of questionable value, since it might
> cause errors and confusion when code is modified. Consider the case
> where you have "SELECT a AS b, b AS a ...". I've done it! :-)
>
> The order of evaluation is: FROM, WHERE, GROUP BY, HAVING, SELECT,
> and, finally, ORDER BY.
Thanks all for these info. & examples, they help me much to clearify
the question, the present wording is a little bit hard to understand, but
accurate and
efficent, and I think it's better to add a few lines to make it clear, like
"...but the SQL standard require (or "only permit") us to do:
select a as b from table1 order by b;"
regards laser
