In this case it's hard to make something fancy :)

    from sqlbind import WHERE, FIELDS, join_fragments

    q = Q()  # a QueryParams factory
    fields = []
    joins = []
    filters = []

    if some_condition:
        fields.append('sub_table.field')
        joins.append('INNER JOIN sub_table ON (subid)')
        filters.append(q.sub_table.date > since)

    sql = f'''\
    SELECT {FIELDS('table.field', *fields)}
    FROM table
    {join_fragments(' ', joins)}
    {WHERE(*filters)}
    '''

Personally I prefer this explicitness to ORM for complex queries which include recursive or multiple joins to the same table.

But I agree ORM shines with simple joins especially if models include proper relationship and there is no need to specify 'ON' condition every time.