Note you can already reference select list items in GROUP, HAVING, and ORDER BY so it's not that big of an extension.
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;
we ended up adding a warning for the case where a select list alias shadowed a table column, suggesting using a fully-qualified table name if they actually wanted the table column (t.x in the above example).
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.
> Note you can already reference select list items in GROUP, HAVING, and ORDER BY so it's not that big of an extension.
You're just looking for symbols in the symbol table, I think it's a big difference!
> IMO only allowing references to previous select list items is a perfectly reasonable...
agreed, see my other post where I say the same.
> Though we did allow using aliases in the WHERE clause
And the SQL standards people didn't go for this, and I'm sure they were very far from stupid. And nobody's asking why they didn't allow this, which really bothers me.
Oh, was your objection specifically to allowing references to following (not just preceding) select list items? Then we're in violent agreement. That would be complicated to implement and confuse users. Definitely not worth it.
> suggesting using a fully-qualified table name if they actually wanted the table column (t.x in the above example).
I just realised why this was bothering me. That means 't' and 't.x' are actually different variables. In standard SQL it's always the case (right?) that an unqualified variable ('t') is just an convenient shorthand for the fully qualified variable ('t.x', or more fully I suppose, '<db>.<schema>.t.x), and you just broke that.
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.
we ended up adding a warning for the case where a select list alias shadowed a table column, suggesting using a fully-qualified table name if they actually wanted the table column (t.x in the above example).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.