Thread: AS operator and subselect result names: PostgreSQL vs. Oracle
AS operator and subselect result names: PostgreSQL vs. Oracle
From
nzanella@cs.mun.ca (Neil Zanella)
Date:
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