AS operator and subselect result names: PostgreSQL vs. Oracle

1. Does standard SQL allow an optional AS keyword for (re/)naming
tables including those resulting from subselects.
Yes.
2 Why must a subselect whose fields are referenced in an outer query
be explicitly named in PostgreSQL when it is not necessary in Oracle.

Hello,

I would like to ask the about the following...

PostgreSQL allows tables resulting from subselects to be renamed with
an optional AS keyword whereas Oracle 9 will report an error whenever
a table is renamed with the AS keyword. Furthermore, in PostgreSQL
when the result of a subselect is referenced in an outer select
it is required that the subselect result be named, whereas this
is not true in Oracle. I wonder what standard SQL has to say
about these two issues. In particular:

1. Does standard SQL allow an optional AS keyword for (re/)naming
tables including those resulting from subselects.

and

2 Why must a subselect whose fields are referenced in an outer query
be explicitly named in PostgreSQL when it is not necessary in Oracle.

PostgreSQL allows tables resulting from subselects to be renamed with
an optional AS keyword whereas Oracle 9 will report an error whenever
a table is renamed with the AS keyword. Furthermore, in PostgreSQL
when the result of a subselect is referenced in an outer select
it is required that the subselect result be named, whereas this
is not true in Oracle. I wonder what standard SQL has to say
about these two issues.
The standard agrees with us.

SQL99 section 7.5 <from clause> says that FROM clause items are
<table reference>s:

<from clause> ::=
FROM <table reference list>

<table reference list> ::=
<table reference> [ { <comma> <table reference> }... ]

the syntax for which appears in 7.6 <table reference>:

<table reference> ::=
<table primary>
| <joined table>

<table primary> ::=
<table or query name> [ [ AS ] <correlation name>
[ <left paren> <derived column list> <right paren> ] ]
| <derived table> [ AS ] <correlation name>
[ <left paren> <derived column list> <right paren> ]
| <lateral derived table> [ AS ] <correlation name>
[ <left paren> <derived column list> <right paren> ]
| <collection derived table> [ AS ] <correlation name>
[ <left paren> <derived column list> <right paren> ]
| <only spec>
[ [ AS ] <correlation name>
[ <left paren> <derived column list> <right paren> ] ]
| <left paren> <joined table> <right paren>

<derived table> ::= <table subquery>

[ I've omitted the definitions for other cases ]

and in 7.14 we find

<table subquery> ::= <subquery>

<subquery> ::=
<left paren> <query expression> <right paren>

So the second alternative (<derived table> ...) is the one that allows a
sub-select.

Notice that the AS-clause ([ AS ] <correlation name> [ <left paren>
<derived column list> <right paren> ]) is bracketed as a whole, making
it optional, in just two of the five alternatives where it appears.
It is required by the syntax in the <derived table> case.

1. Does standard SQL allow an optional AS keyword for (re/)naming
tables including those resulting from subselects.
It does not "allow" it, it requires it.
2 Why must a subselect whose fields are referenced in an outer query
be explicitly named in PostgreSQL when it is not necessary in Oracle.