Require that nested joins always have one join

The SQL specification prohibits constructions like

    SELECT * FROM a NATURAL JOIN (b)

where b sits alone inside parentheses. Parentheses in a FROM entry
always introduce either a derived table or a join.

src/sqlast/query.rs

@@ -238,6 +238,10 @@ 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

@@ -1719,6 +1719,16 @@ impl Parser {
                     // we won't, and we'll return that error instead.
                     self.index = index;
                     let table_and_joins = self.parse_table_and_joins()?;
+                    match table_and_joins.relation {
+                        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)))
                 }

tests/sqlparser_common.rs

@@ -1742,6 +1742,12 @@ fn parse_join_nesting() {
         from.joins,
         vec![join(nest!(nest!(nest!(table("b"), table("c")))))]
     );
+
+    let res = parse_sql_statements("SELECT * FROM (a NATURAL JOIN (b))");
+    assert_eq!(
+        ParserError::ParserError("Expected joined table, found: )".to_string()),
+        res.unwrap_err()
+    );
 }
 
 #[test]
@@ -1873,7 +1879,13 @@ fn parse_derived_tables() {
                 join_operator: JoinOperator::Inner(JoinConstraint::Natural),
             }],
         }))
-    )
+    );
+
+    let res = parse_sql_statements("SELECT * FROM ((SELECT 1) AS t)");
+    assert_eq!(
+        ParserError::ParserError("Expected joined table, found: )".to_string()),
+        res.unwrap_err()
+    );
 }
 
 #[test]