$list=db('shop_goods')->alias('a')->
join('task b','b.shop_name_id=a.num_iid')->
join('task_goods c','c.gid=a.num_iid')->
where('c.member','gt',0)->
select();
还是
$list=db('shop_goods')->alias('a')->
join('task b','b.shop_name_id=a.num_iid')->
join('task_goods c','c.gid=a.num_iid')->
where('c.member > 0')->
select();
都查询不到，请问多表查询的where该怎么写