code: function _addColumn(){
      self._visitingAlter = true;
      var table = self._queryNode.table;
      self._visitingAddColumn = true;
      var result='ALTER TABLE '+self.visit(table.toNode())+' ADD ('+self.visit(alter.nodes[0].nodes[0]);
      for (var i= 1,len=alter.nodes.length; i<len; i++){
        var node=alter.nodes[i];
        assert(node.type=='ADD COLUMN',errMsg);
        result+=', '+self.visit(node.nodes[0]);
      }
      result+=')';
      self._visitingAddColumn = false;
      self._visitingAlter = false;
      return [result];
    }
docs: Implement our own add column: PostgreSQL: ALTER TABLE "name" ADD COLUMN "col1", ADD COLUMN "col2" Oracle:  ALTER TABLE "name" ADD ("col1", "col2")