Re: SQL query syntax question

On Monday, April 4, 2016, Alexander Levsha <levsha(dot)alexander(at)gmail(dot)com>
wrote:

> Hello all.
> I'm currently developing an SQL parser/analyzer for internal PostgreSQL
> tool we use at my workplace.
> Naturally, i use psql in conjunction with PostgreSQL documentation to
> develop and verify my work.
>
> Recently i've noticed the following oddity in query syntax/execution:
>
> select (select c1 from tt) from t1 tt;
> --ERROR: relation "tt" does not exist
> --LINE 1: select (select c1 from tt) from t1 tt;
>
>
> select (select tt.c1 ) from t1 tt;
> --c1
> ------
> --(0 rows)
>
>
> Why doesn't the first one work when the second one works fine? In other
> words, why do subquery's traget list and range table list use different
> namespaces to resolve table references/aliases? Is this an unintended
> behaviour, implementation detail or is there an actual reason for this?
>
> Cannot speak to restrictions imposed by the SQL standard but the exhibited
behavior seems logical given the nature of subqueries.

The relation named in the FROM clause must exist in the database schema or
have been previously "created" using a CTE. Within a subquery all from
clause entries behave the same but you can reference a column by name (with
usually optional table and schema prefix) if it exists in the containing
scope. Thus there is no need to complicate things by requiring (or
allowing) the outer relation names to be targeted by a FROM clause in a
subquery.

Correlated subqueries require target list resolution to behave in that
manner. It doesn't seem useful to complicate range table resolution
lacking a similar need.

What is the motivation for the inquiry?

David J.