MySQL:
select * from t order by IF(ISNULL(order_col),1,0), order_col asc;

Oracle
select * from t order by order_col asc nulls last;  

用Union 合并查询结果。
(select * from a where service_time = !null order by service_time asc)
union
(select * from a where service_time = null);

where 后条件 按你具体的service_time 默认值做相应修改即可。