improve output visitor

src/main/java/com/alibaba/druid/sql/visitor/ParameterizedOutputVisitorUtils.java

@@ -189,6 +189,12 @@ public class ParameterizedOutputVisitorUtils {
         v.incrementReplaceCunt();
         return false;
     }
+    
+    public static boolean visit(ParameterizedVisitor v, SQLVariantRefExpr x) {
+        v.print('?');
+        v.incrementReplaceCunt();
+        return false;
+    }
 
     public static SQLBinaryOpExpr merge(ParameterizedVisitor v, SQLBinaryOpExpr x) {
         SQLExpr left = x.getLeft();

src/main/java/com/alibaba/druid/sql/visitor/SQLASTOutputVisitor.java

@@ -601,7 +601,9 @@ public class SQLASTOutputVisitor extends SQLASTVisitorAdapter implements Printab
             decrementIndent();
         } else if (parent instanceof ValuesClause) {
             println();
+            print("(");
             x.getSubQuery().accept(this);
+            print(")");
             println();
         } else {
             print("(");