code: public function addSelect($columns)
      {
          if ($columns instanceof ExpressionInterface) {
              $columns = [$columns];
          } elseif (!is_array($columns)) {
              $columns = preg_split('/\s*,\s*/', trim($columns), -1, PREG_SPLIT_NO_EMPTY);
          }
          $columns = $this->getUniqueColumns($columns);
          if ($this->select === null) {
              $this->select = $columns;
          } else {
              $this->select = array_merge($this->select, $columns);
          }
  
          return $this;
      }
docs: Add more columns to the SELECT part of the query.
  
  Note, that if [[select]] has not been specified before, you should include `*` explicitly
  if you want to select all remaining columns too:
  
  ```php
  $query->addSelect(["*", "CONCAT(first_name, ' ', last_name) AS full_name"])->one();
  ```
  
  @param string|array|ExpressionInterface $columns the columns to add to the select. See [[select()]] for more
  details about the format of this parameter.
  @return $this the query object itself
  @see select()