code: function _union() {
      var onlyUnions = this.onlyUnions();
      var unions = this.grouped.union;
      if (!unions) {
        return '';
      }
      var sql = '';
      for (var i = 0, l = unions.length; i < l; i++) {
        var union = unions[i];
        if (i > 0) {
          sql += ' ';
        }
        if (i > 0 || !onlyUnions) {
          sql += union.clause + ' ';
        }
        var statement = this.formatter.rawOrFn(union.value);
        if (statement) {
          if (union.wrap) {
            sql += '(';
          }
          sql += statement;
          if (union.wrap) {
            sql += ')';
          }
        }
      }
      return sql;
    }
docs: Compile the "union" queries attached to the main query.