Question

 var query = from r in db.Resource
                    join c in db.ResourceProjectedCapacity on r.ID equals c.ResourceID into ps
                    from c in ps.DefaultIfEmpty(null)
                    join p in db.Project on c.ProjectID equals p.ID
                    select new
                    {
                        Capacity = c,
                        Resource = r,
                        Project = p
                    };

我有这个linq查询，但它只返回在ResourceProjectedCapacity表上有匹配行的资源。如何获取所有资源，如果没有匹配的记录，则Capacity对象为null？

Answer 1

from i in db.Resource
let c = db.ResourceProjectedCapacity.Where(cc => i.id == cc.ResourceID).FirstOrDefault()
let p = db.Project.Where(pp => c.ProjectID == pp.ID).FirstOrDefault() 
select new
{
  Capacity = C,
 Resource = i,
 Project = p
 }

尝试上面的代码

Answer 2

我认为次要内部连接会弄乱它上方的左外连接。我认为解决这个问题的一种方法是将连接分解为单独的查询，然后将连接保留在该连接上，如下所示：

var subquery = from c in db.ResourceProjectedCapacity
               join p in db.Project on c.ProjectID equals p.ID
               select new { c, p };


var query = from r in db.Resource
                    join c in subquery on r.ID equals c.c.ResourceID into ps
                    from c in ps.DefaultIfEmpty(null)
                    select new
                    {
                        Capacity = c.c,
                        Resource = r,
                        Project = c.p
                    };

请不要担心，它不会执行两个数据库查询，只有在使用query评估.ToList()之后，它才会针对您的数据库执行这一点。

如何编写LINQ左连接

2 个答案: