Обсуждение: AS operator and subselect result names: PostgreSQL vs. 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.

Thanks,

Neil

Neil Zanella writes:

> 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.

Because the SQL standard says so.

--
Peter Eisentraut   peter_e@gmx.net

On Fri, 31 Oct 2003, Neil Zanella wrote:

> 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.

I believe the section in question of SQL92 that you're asking about
says explicitly that a table reference from a derived table should look
like:
<derived table> [ AS ] <correlation name> [ <left paren> <derived column
list> <right paren> ]
where <derived table> is a table subquery.

It's possible that SQL99 changes this, but in SQL92 at least, it looks
like the correlation name is not optional (although the AS keyword is).

nzanella@cs.mun.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