var query=db.UserPay.where(u=>true);
var queryList=from u in query
where (from u in db.Users select u.id).Contains(u.UserId)
select u;
//等效于如下sql语句
select * from UserPay where userid in (select id from users)
var query=db.UserPay.where(u=>true);
var queryList=from u in query
where !(from u in db.Users select u.id).Contains(u.UserId)
select u;
//等效于如下sql语句
select * from UserPay where userid not in (select id from users)
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