Question

可能重复：
How do you perform a left outer join using linq extension methods

我找不到Linq lambda的左外连接示例（使用扩展方法），至少不是一个明确的。

假设我有下表：

Parent
{
    PID     // PK
}

Child
{
    CID     // PK
    PID     // FK
    Text
}

我想加入Parent with Child，对于每个失踪的孩子，我希望Text的默认值为“[[Empty]]”。我怎么能用linq lambda语法做到这一点？

我目前有以下内容：

var source = lParent.GroupJoin(
    lChild,
    p => p.PID,
    c => c.PID,
    (p, g) =>
        new // ParentChildJoined
        {
            PID = p.PID;
            // How do I add child values here?
        });

Answer 1

你很亲密。以下内容将为每个孩子选择PID，CID和Text，并为没有孩子的每位家长选择PID，CID = -1和Text = "[[Empty]]"：

var source = lParent.GroupJoin(
    lChild,
    p => p.PID,
    c => c.PID,
    (p, g) => g
        .Select(c => new { PID = p.PID, CID = c.CID, Text = c.Text })
        .DefaultIfEmpty(new { PID = p.PID, CID = -1, Text = "[[Empty]]" }))
    .SelectMany(g => g);

Answer 2

from p in Parent
join c in Child on p.PID equals c.PID into g
from c in g.DefaultIfEmpty
select new 
{
   p.PID,
   CID = c != null ? (int?)c.CID : null, // Could be null
   Text = c != null ? c.Text : "[[Empty]]"
}

使用lambda：

class ChildResult
{
   public int PID { get; set; }
   public int? CID { get; set; }
   public string Text { get; set; }
}

lParent.SelectMany(p => p.Childs.Any() ?
  p.Childs.Select(c => new ChildResult() { PID = c.PID, CID = c.CID, Text = c.Text }) :
  new [] { new ChildResult() { PID = p.PID, CID = null, Text = "[[Empty]]" } } );

Linq中的lambda /方法语法中的左外连接

2 个答案: