But I know a user requirement when I hear one, so can you give me an large, real example of where allowing this would make things easier? That would be mega helpful, ta
No, it is not. I mean it is, but not in parts where that could be seen as useful and/or convenient. [A]cyclic graph traversal/etc is one of the basic tests in a modern interview at any CRUD studio. How come it could do $%^& to any part of yours?
Because just implementing the standard stuff nearly did my $^&% nut. Also I know about graphs & posets, and it's potentially a little more complex than it seems. The variables
select x * x as y, 1 as x
select
(select tbl.z from tbl where tbl.y = y) as subq,
x * yy as y,
xx + 1 as x,
subq + yy as zzz
from (
select xx, yy
from ... )
I've implemented the ability to reference select-list aliases before; it's not that hard to do if implemented basically like a macro expansion. The main problem is user confusion due to ambiguous references, e.g.
select 2 as x, x as `which x?`
from (select 1 as x) t;
IMO only allowing references to previous select list items is a perfectly reasonable restriction; loosening it isn't worth the implementation headache or user confusion. Though we did allow using aliases in the WHERE clause.
That’s no different than the first snippet, if you aren’t parsing it with regexps, of course. The resulting AST identifiers would simply refer to not only column names, but also to other defined expressions. This is the case for both snippets. It’s either cyclic or not, and when not, it is easy to substitute/cse/etc as usual. The complexity of these expressions is irrelevant.
I can do that I just don't fancy it, and more to the point nobody is giving me an example of where it would be particularly helpful. So if anyone can, I'm interested.
(also, consider human factors; although an acyclic definition could be extracted from an unordered expression set, a consistent left to right (in the western world anyway, matching textual layout) with dependencies being introduced on the right and depending only on what came before on the left might actually be better for us meatsacks)
select id, count(...something complicated) as complicated_count
from ....
order by complicated_count
This isn't really the large, convincing example I was looking for btw.
IMO it's most useful (though somewhat more difficult to implement) to be able to use the aliases with window functions or large case/when statements, something like
SELECT
page,
SUM(clicks) AS total_clicks,
100. * total_clicks / (SUM(total_clicks) OVER ()) AS click_pct,
100. * SUM(total_clicks) OVER (ORDER BY total_clicks DESC ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) / (SUM(total_clicks) OVER ()) AS cumulative_click_pct
FROM weblog
GROUP BY page;