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.

Author: benesch

src/sqlparser.rs

@@ -1708,6 +1708,16 @@ impl Parser {
                     // error instead.
                     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())?
+                        }
+                        _ => (),
+                    }
                     self.expect_token(&Token::RParen)?;
                     Ok(TableFactor::NestedJoin(Box::new(table_and_joins)))
                 }

tests/sqlparser_common.rs

@@ -1731,6 +1731,12 @@ 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]
@@ -1862,7 +1868,13 @@ 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]