Gets a column's value for a specific record.

    Args:
        table: A string - name of the database table.
        record: A string - identifier of the record.
        column: A string - name of the column.
        if_exists: A boolean - if True, it is not an error if the record does
            not exist.

    Returns:
        The column's value.

    CLI Example:
    .. code-block:: bash

       salt '*' openvswitch.db_get Port br0 vlan_mode

def db_get(table, record, column, if_exists=False):
    '''
    Gets a column's value for a specific record.

    Args:
        table: A string - name of the database table.
        record: A string - identifier of the record.
        column: A string - name of the column.
        if_exists: A boolean - if True, it is not an error if the record does
            not exist.

    Returns:
        The column's value.

    CLI Example:
    .. code-block:: bash

       salt '*' openvswitch.db_get Port br0 vlan_mode
    '''
    cmd = ['ovs-vsctl', '--format=json', '--columns={0}'.format(column)]
    if if_exists:
        cmd += ['--if-exists']
    cmd += ['list', table, record]
    result = __salt__['cmd.run_all'](cmd)
    if result['retcode'] != 0:
        raise CommandExecutionError(result['stderr'])
    output = _stdout_parse_json(result['stdout'])
    if output['data'] and output['data'][0]:
        return output['data'][0][0]
    else:
        return None