code: public static function statement(Database $database, $modelName, $tableName, array $tableDefinition, $uniqueName)
      {
          $uniqueDefinition = $tableDefinition['uniques'][$uniqueName];
          $fields = array();
          foreach ($uniqueDefinition as $field) {
              if (isset($tableDefinition['fields'][$field]['name']) === true) {
                  $fields[] = $tableDefinition['fields'][$field]['name'];
              } else {
                  $fields[] = Helper::codifyName($modelName) . '_' . Helper::codifyName($field);
              }
          }
          $statement = $database->getMasterLink()->prepare('ALTER TABLE ' . $tableName . ' ADD CONSTRAINT ' . $uniqueName . ' UNIQUE (' . implode(', ', $fields) . ')');
  
          return $statement;
      }
docs: Prepare and return according PDO statement.
  
  @param  Database     $database         database
  @param  string       $modelName        model name
  @param  string       $tableName        table real name
  @param  array        $tableDefinition  table definition
  @param  string       $uniqueName       unique constraint name
  @return \PDOStatement $statement prepared query