Re: AS operator and subselect result names: PostgreSQL vs. Oracle

nzanella(at)cs(dot)mun(dot)ca (Neil Zanella) writes:
> 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.

We insist on a name because otherwise we'd have to invent a name for the
FROM-clause item, and in most cases there's not an obvious choice for a
default name. I dunno what Oracle does about choosing a name, but it's
not standard behavior.

regards, tom lane