def set_property(cursor, entity_id: int, property_name: str):
    """
    Adds a property for the entity. If it already exists, does nothing.
    """
    cursor.execute("""
        INSERT INTO property (id, property_name)
        VALUES (%s, %s)
        ON CONFLICT (id, property_name) DO NOTHING
    """, (entity_id, property_name))

def delete_property(cursor, entity_id, property_name):
    """
    Remove a property from an entity.
    Raises ValueError if the property does not exist.
    """
    cursor.execute(
        "DELETE FROM property WHERE id=%s AND property_name=%s",
        (entity_id, property_name)
    )
    if cursor.rowcount == 0:
        raise ValueError("Property not found")

def get_properties(cursor, entity_id: int):
    """
    Returns a list of property names for the given entity.
    """
    cursor.execute("""
        SELECT property_name
        FROM property
        WHERE id = %s
    """, (entity_id,))
    return [row['property_name'] for row in cursor.fetchall()]