framework/db/Query.php
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 $this;
    }
public function addSelect($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;
    }
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()