postgresql - Why is SELECT without columns valid

Question

I accidently wrote a query like select from my_table; and surprisingly it is valid statement. Even more interesting to me is that even SELECT; is a valid query in PostgreSQL. You can try to write a lot funny queries with this:

select union all select;
with t as (select) select;
select from (select) a, (select) b;
select where exists (select);
create table a (b int); with t as (select) insert into a (select from t);

Is this a consequence of some definition SQL standard, or there is some use case for it, or it is just funny behavior that no one cared to programatically restrict?

Answer 1

Right from the manual:

The list of output expressions after SELECT can be empty, producing a zero-column result table. This is not valid syntax according to the SQL standard. PostgreSQL allows it to be consistent with allowing zero-column tables. However, an empty list is not allowed when DISTINCT is used.

The possibility of "zero-column" tables is a side effect of the table inheritance if I'm not mistaken. There were discussions over this on the Postgres mailing lists (but I can't find them right now)