查询 users 表中存在而 orders 表中不存在的 user_id

A、selectusers.user_idfromusersleftjoinordersonusers.user_id=orders.user_id;
B、selectusers.user_idfromusersrightjoinordersonusers.user_id=orders.user_id;
C、selectusers.user_idfromusersleftjoinordersonusers.user_id=orders.user_idwhereorders.user_idisnull;
D、selectusers.user_idfromusersrightjoinordersonusers.user_id=orders.user_idwhereusers.user_idisnull;