code: public function select($select)
      {
          if (is_array($select) === true) {
              $this->select = '';
              foreach ($select as $key => $elem) {
                  $this->select .= $elem;
                  if (is_numeric($key) === false) {
                      $this->select .= ' AS ' . $key;
                  }
                  $this->select .= ', ';
              }
              $this->select = substr($this->select, 0, -2);
          } elseif (! is_string($select)) {
              throw new InvalidArgumentException(sprintf(
                  'Argument 1 passed to %s must be an array or string!',
                  __METHOD__
              ));
          } else {
              $this->select = $select;
          }
  
          return $this;
      }
docs: Set select clause.
  
  @param  array|string $select select clause
  @return Select       this