public function addPrimaryKey($columns)
    {
        $schema = self::SELECT.' EXISTS
                   (
                       '.self::SELECT.' * FROM '.$this->_informationSchema.'.COLUMNS
                       WHERE '.$this->_tableSchema."= '".$this->database."' AND
                       TABLE_NAME ='".$this->tableName."' AND
                       COLUMN_KEY = 'PRI'

                   ) AS has_primary_key;";

        $hasPrimaryKey = $this->_connection->prepare($schema)->execute();

        if ($hasPrimaryKey === true) {
            $query = '';
            $query = self::ALTER_TABLE.'`'.$this->tableName.'` CHANGE
             `'.$this->primaryKey.'` `'.$this->primaryKey.'` INT( 11 ) NOT NULL';
            $primaryKey = $this->_connection->prepare($query)->execute();

            $schemaString = '';
            $schemaString = $this->commands($columns);

            $this->schema = static::ALTER_TABLE.' `'.$this->database.'`.`'.$this->tableName.'` DROP PRIMARY KEY,
                ADD CONSTRAINT PK_'.strtoupper($this->tableName).'_ID
                PRIMARY KEY ('.$schemaString.')';
        } else {
            $this->schema = static::ALTER_TABLE.' `'.$this->database.'`.`'.$this->tableName.'` ADD
            '.'PRIMARY KEY ('.$columns.')';
        }

        return $this;
    }