join constraints are not required

Author: donhcd

src/ast/query.rs

@@ -471,6 +471,7 @@ pub enum JoinConstraint {
     Where(Expr),
     Using(Vec<Ident>),
     Natural,
+    Empty,
 }
 
 /// An `ORDER BY` expression

src/parser.rs

@@ -2471,7 +2471,7 @@ impl<'a> Parser<'a> {
             let columns = self.parse_parenthesized_column_list(Mandatory)?;
             Ok(JoinConstraint::Using(columns))
         } else {
-            self.expected("ON, or USING after JOIN", self.peek_token())
+            Ok(JoinConstraint::Empty)
         }
     }