code: function _columns() {
      var distinct = false;
      if (this.onlyUnions()) {
        return '';
      }
      var columns = this.grouped.columns || [];
      var i = -1,
          sql = [];
      if (columns) {
        while (++i < columns.length) {
          var stmt = columns[i];
          if (stmt.distinct) {
            distinct = true;
          }
          if (stmt.type === 'aggregate') {
            sql.push(this.aggregate(stmt));
          } else if (stmt.value && stmt.value.length > 0) {
            sql.push(this.formatter.columnize(stmt.value));
          }
        }
      }
      if (sql.length === 0) {
        sql = ['*'];
      }
      return 'select ' + (distinct ? 'distinct ' : '') + sql.join(', ') + (this.tableName ? ' from ' + this.tableName : '');
    }
docs: Compiles the columns in the query, specifying if an item was distinct.