Allow no joins only for TableFactor::NestedJoin

...instead of listing all the table factor variants that must be
followed by joins when parenthesized. This way is closer to the spec --
see `<parenthesized joined table>` definition
https://jakewheat.github.io/sql-overview/sql-2011-foundation-grammar.html#parenthesized-joined-table

Author: nickolay

src/sqlast/query.rs

@@ -226,6 +226,10 @@ pub enum TableFactor {
         subquery: Box<SQLQuery>,
         alias: Option<TableAlias>,
     },
+    /// Represents a parenthesized join expression, such as
+    /// `(foo <JOIN> bar [ <JOIN> baz ... ])`.
+    /// The inner `TableWithJoins` can have no joins only if its
+    /// `relation` is itself a `TableFactor::NestedJoin`.
     NestedJoin(Box<TableWithJoins>),
 }
 

src/sqlparser.rs

@@ -1712,14 +1712,14 @@ impl Parser {
                     self.index = index;
                     let table_and_joins = self.parse_table_and_joins()?;
                     match table_and_joins.relation {
-                        TableFactor::Table { .. } | TableFactor::Derived { .. }
-                            if table_and_joins.joins.is_empty() =>
-                        {
-                            // The SQL spec prohibits derived tables and bare
-                            // tables from appearing alone in parentheses.
-                            self.expected("joined table", self.peek_token())?
+                        TableFactor::NestedJoin { .. } => (),
+                        _ => {
+                            if table_and_joins.joins.is_empty() {
+                                // The SQL spec prohibits derived tables and bare
+                                // tables from appearing alone in parentheses.
+                                self.expected("joined table", self.peek_token())?
+                            }
                         }
-                        _ => (),
                     }
                     self.expect_token(&Token::RParen)?;
                     Ok(TableFactor::NestedJoin(Box::new(table_and_joins)))