linq-与select max左连接，并检查记录是否不为null

Question

I have 3 tables:

• customers
• users
• payments

I need to select customers join with users, left join with payments. Sometimes I need to check if max record from payment is not null.

I wrote this but it crashes:

I get parameter to this function - byCard

from c in _db.Customers
from p in _db.Payments.Where(r => r.CustId == c.ID && r.Success).OrderBy(r =>   r.PaymentDate).Take(1).DefaultIfEmpty()
join u1 in _db.Users on c.UserID equals u1.Id into us
     from use1 in us.DefaultIfEmpty()
&& (byCard == "1" ? p!=null : true)

Answer 1

您不应该使用OrderByDescending代替最大的吗？