code: public function getSql()
      {
          if ($this->column instanceof CustomColumn) {
              $columnType = $this->column->getType();
          } else {
              $columnTypeMapper = new ColumnTypeMapper();
              $columnType = $columnTypeMapper->getNative($this->column->getType());
          }
  
           $sql = sprintf('ALTER TABLE %s ADD COLUMN %s %s %s %s;'
              , $this->column->getTable()->getQualifiedName()
              , $this->column->getName()
              , $columnType
              , $this->column->isNotNull() ? 'NOT NULL' : ''
              , null === $this->column->getDefault() ? '' : 'DEFAULT'. ' ' . $this->column->getDefault()
          );
  
          $columnDescription = $this->column->getDescription();
          if (!empty($columnDescription)) {
              $sql .= sprintf("COMMENT ON COLUMN %s.%s IS '%s';",
                  $this->column->getTable()->getQualifiedName(),
                  $this->column->getName(),
                  $columnDescription);
          }
  
          return $sql;
      }
docs: {@inheritdoc}