Re: Nested JOINs

"Josh Berkus" <josh(at)agliodbs(dot)com> writes:
> Second, you can't alias a JOINed set of tables;

Actually you can, according to my reading of SQL92:

<table reference> ::=
<table name> [ [ AS ] <correlation name>
[ <left paren> <derived column list> <right paren> ] ]
| <derived table> [ AS ] <correlation name>
[ <left paren> <derived column list> <right paren> ]
| <joined table>

<derived table> ::= <table subquery>

<table subquery> ::= <subquery>

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

<query expression> ::=
<non-join query expression>
| <joined table>

<joined table> ::=
<cross join>
| <qualified join>
| <left paren> <joined table> <right paren>

<qualified join> ::=
<table reference> [ NATURAL ] [ <join type> ] JOIN
<table reference> [ <join specification> ]

So a parenthesized <qualified join> is a <derived table> and may
be followed by [ AS ] <correlation name>.

7.1 gets this right. 7.0's very preliminary implementation of JOIN
syntax did not. (It had a number of other bugs too, IIRC.)

The spec's grammar is horribly badly written: it's ambiguous whether the
parentheses should be parsed as part of a second-level <joined table>
which would imply that a correlation name would NOT be accepted.
It took a fair amount of work to derive a grammar that was unambiguous
and still accepted everything...

regards, tom lane