c# - LInq left join with multiple condition in on 子句

Question

我在 SQL 中有一个连接，我需要在 linq 中编写它，这里是 select 语句:

select * from products p
left join customer cust
on((cust.identifier = p.orgID or cust.identifier = p.personalno) and cust.identifier is not null);

如何在 linq 中编写此语句？

Answer 1

• 如果你真的想用JOIN来做，你可以这样做:

  from cust in customer
  join _p1 in products on cust.identifier equals _p1.orgID into _p1
  from p1 in _p1.DefaultIfEmpty()
  join _p2 in products on cust.identifier equals _p2.personalno into _p2
  from p2 in _p2.DefaultIfEmpty()
  where cust.identifier != null
  && (p1 != null || p2 != null)
  select new { Customer = cust, Product = p1 == null ? p2 : p1 }
  

• 没有 JOIN 关键字的更简单的解决方案是:

  from p in products
  from cust.Where(q => q.identifier != null && (p.orgID == q.identifier || p.personalno == q.identifier)).DefaultIfEmpty()
  where cust != null
  select new { Product = p, Customer = cust }
  

• 要回答标题建议的问题，请执行以下操作以在多个条件下进行连接，但请注意，这仅适用于 AND 条件，不适用于 OR 条件:

  from t1 in Table1
  join t2 in Table2 on new { Criteria1 = t1.criteria1, Criteria2 = t1.criteria2 } equals new { Criteria1 = t2.criteria1, Criteria2 = t2.criteria2 }
  select new { T1 = t1, T2 = t2 }