public function addPrimaryKey($name, $table, $columns)
    {
        if (is_string($columns)) {
            $columns = preg_split('/\s*,\s*/', $columns, -1, PREG_SPLIT_NO_EMPTY);
        }
        foreach ($columns as $i => $col) {
            $columns[$i] = $this->quoteColumnName($col);
        }

        return
            'ALTER TABLE ' .
            $this->quoteTableName($table) .
            ' ADD CONSTRAINT ' .
            $this->quoteColumnName($name) .
            '  PRIMARY KEY (' .
            implode(', ', $columns) .
            ' )';
    }