[snowflake] Support specifying an alias after FROM (table_factor)

Snowflake diverges from the standard and from most of the other
implementations by allowing extra parentheses not only around a join,
but around lone table names (e.g. `FROM (mytable [AS alias])`) and
around derived tables (e.g. `FROM ((SELECT ...)  [AS alias])`) as well.

Initially this was implemented in https://github.com/ballista-compute/sqlparser-rs/issues/154
by (ab)using `TableFactor::NestedJoin` to represent anything nested in
extra set of parens.

Afterwards we learned in https://github.com/ballista-compute/sqlparser-rs/issues/223
that in cases of such extraneous nesting Snowflake allows specifying the
alias both inside and outside parens, but not both - consider:

    FROM (table_factor AS inner_alias) AS outer_alias

We've considered implementing this by changing `TableFactor::NestedJoin`
to a `TableFactor::Nested { inner: TableWithJoins, alias:
Option<TableAlias> }`, but that seemed too generic, as no known dialect
supports duplicate aliases, as shown above, nor naming nested joins
`(foo NATURAL JOIN bar) alias`. So we decided on making a smaller change
(with no modifications to the AST), that is also more appropriate to the
contributors to the Snowflake dialect:


1) Revert #154 by rejecting `FROM (table or derived table)` in most dialects.

2) For `dialect_of!(self is SnowflakeDialect | GenericDialect)` parse
and strip the extraneous parentheses, e.g.

   `(mytable) AS alias` -> `(mytable AS alias)`


Co-authored-by: Eyal Leshem <eyal@satoricyber.com>

src/parser.rs

@@ -2156,14 +2156,58 @@ impl<'a> Parser<'a> {
             // recently consumed does not start a derived table (cases 1, 2, or 4).
             // `maybe_parse` will ignore such an error and rewind to be after the opening '('.
 
-            // Inside the parentheses we expect to find a table factor
-            // followed by some joins or another level of nesting.
-            let table_and_joins = self.parse_table_and_joins()?;
-            self.expect_token(&Token::RParen)?;
-            // The SQL spec prohibits derived and bare tables from appearing
-            // alone in parentheses. We don't enforce this as some databases
-            // (e.g. Snowflake) allow such syntax.
-            Ok(TableFactor::NestedJoin(Box::new(table_and_joins)))
+            // Inside the parentheses we expect to find an (A) table factor
+            // followed by some joins or (B) another level of nesting.
+            let mut table_and_joins = self.parse_table_and_joins()?;
+
+            if !table_and_joins.joins.is_empty() {
+                self.expect_token(&Token::RParen)?;
+                Ok(TableFactor::NestedJoin(Box::new(table_and_joins))) // (A)
+            } else if let TableFactor::NestedJoin(_) = &table_and_joins.relation {
+                // (B): `table_and_joins` (what we found inside the parentheses)
+                // is a nested join `(foo JOIN bar)`, not followed by other joins.
+                self.expect_token(&Token::RParen)?;
+                Ok(TableFactor::NestedJoin(Box::new(table_and_joins)))
+            } else if dialect_of!(self is SnowflakeDialect | GenericDialect) {
+                // Dialect-specific behavior: Snowflake diverges from the
+                // standard and from most of the other implementations by
+                // allowing extra parentheses not only around a join (B), but
+                // around lone table names (e.g. `FROM (mytable [AS alias])`)
+                // and around derived tables (e.g. `FROM ((SELECT ...)
+                // [AS alias])`) as well.
+                self.expect_token(&Token::RParen)?;
+
+                if let Some(outer_alias) =
+                    self.parse_optional_table_alias(keywords::RESERVED_FOR_TABLE_ALIAS)?
+                {
+                    // Snowflake also allows specifying an alias *after* parens
+                    // e.g. `FROM (mytable) AS alias`
+                    match &mut table_and_joins.relation {
+                        TableFactor::Derived { alias, .. }
+                        | TableFactor::Table { alias, .. }
+                        | TableFactor::TableFunction { alias, .. } => {
+                            // but not `FROM (mytable AS alias1) AS alias2`.
+                            if let Some(inner_alias) = alias {
+                                return Err(ParserError::ParserError(format!(
+                                    "duplicate alias {}",
+                                    inner_alias
+                                )));
+                            }
+                            // Act as if the alias was specified normally next
+                            // to the table name: `(mytable) AS alias` ->
+                            // `(mytable AS alias)`
+                            alias.replace(outer_alias);
+                        }
+                        TableFactor::NestedJoin(_) => unreachable!(),
+                    };
+                }
+                // Do not store the extra set of parens in the AST
+                Ok(table_and_joins.relation)
+            } else {
+                // The SQL spec prohibits derived tables and bare tables from
+                // appearing alone in parentheses (e.g. `FROM (mytable)`)
+                self.expected("joined table", self.peek_token())
+            }
         } else {
             let name = self.parse_object_name()?;
             // Postgres, MSSQL: table-valued functions: