如何使用 linq 扩展方法执行左外连接

对于表 Bar 与 Foo.Foo_Id = Bar.Foo_Id 上的表 Foo 的（左外）联接，采用 lambda 表示法：

var qry = Foo.GroupJoin(
          Bar, 
          foo => foo.Foo_Id,
          bar => bar.Foo_Id,
          (x,y) => new { Foo = x, Bars = y })
       .SelectMany(
           x => x.Bars.DefaultIfEmpty(),
           (x,y) => new { Foo=x.Foo, Bar=y});

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由于这似乎是使用方法（扩展）语法的左外连接的事实上的 SO 问题，我想我会为当前选择的答案添加一个替代方案（至少在我的经验中）更常见的是我后

// Option 1: Expecting either 0 or 1 matches from the "Right"
// table (Bars in this case):
var qry = Foos.GroupJoin(
          Bars,
          foo => foo.Foo_Id,
          bar => bar.Foo_Id,
          (f,bs) => new { Foo = f, Bar = bs.SingleOrDefault() });

// Option 2: Expecting either 0 or more matches from the "Right" table
// (courtesy of currently selected answer):
var qry = Foos.GroupJoin(
                  Bars, 
                  foo => foo.Foo_Id,
                  bar => bar.Foo_Id,
                  (f,bs) => new { Foo = f, Bars = bs })
              .SelectMany(
                  fooBars => fooBars.Bars.DefaultIfEmpty(),
                  (x,y) => new { Foo = x.Foo, Bar = y });

使用简单的数据集显示差异（假设我们正在加入值本身）：

List<int> tableA = new List<int> { 1, 2, 3 };
List<int?> tableB = new List<int?> { 3, 4, 5 };

// Result using both Option 1 and 2. Option 1 would be a better choice
// if we didn't expect multiple matches in tableB.
{ A = 1, B = null }
{ A = 2, B = null }
{ A = 3, B = 3    }

List<int> tableA = new List<int> { 1, 2, 3 };
List<int?> tableB = new List<int?> { 3, 3, 4 };

// Result using Option 1 would be that an exception gets thrown on
// SingleOrDefault(), but if we use FirstOrDefault() instead to illustrate:
{ A = 1, B = null }
{ A = 2, B = null }
{ A = 3, B = 3    } // Misleading, we had multiple matches.
                    // Which 3 should get selected (not arbitrarily the first)?.

// Result using Option 2:
{ A = 1, B = null }
{ A = 2, B = null }
{ A = 3, B = 3    }
{ A = 3, B = 3    }    

选项 2 适用于典型的左外连接定义，但正如我之前提到的，根据数据集的不同，它通常过于复杂。

---

Group Join 方法对于实现两个数据集的连接是不必要的。

内部联接：

var qry = Foos.SelectMany
            (
                foo => Bars.Where (bar => foo.Foo_id == bar.Foo_id),
                (foo, bar) => new
                    {
                    Foo = foo,
                    Bar = bar
                    }
            );

对于左连接，只需添加 DefaultIfEmpty()

var qry = Foos.SelectMany
            (
                foo => Bars.Where (bar => foo.Foo_id == bar.Foo_id).DefaultIfEmpty(),
                (foo, bar) => new
                    {
                    Foo = foo,
                    Bar = bar
                    }
            );

EF 和 LINQ to SQL 正确转换为 SQL。对于 LINQ to Objects，最好使用 GroupJoin 加入，因为它在内部使用 Lookup。但是，如果您正在查询数据库，那么跳过 GroupJoin 是 AFAIK 的表现。

与 GroupJoin().SelectMany() 相比，这种方式对我来说 Personlay 更具可读性

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您可以创建扩展方法，例如：

public static IEnumerable<TResult> LeftOuterJoin<TSource, TInner, TKey, TResult>(this IEnumerable<TSource> source, IEnumerable<TInner> other, Func<TSource, TKey> func, Func<TInner, TKey> innerkey, Func<TSource, TInner, TResult> res)
    {
        return from f in source
               join b in other on func.Invoke(f) equals innerkey.Invoke(b) into g
               from result in g.DefaultIfEmpty()
               select res.Invoke(f, result);
    }

---

改进 Ocelot20 的答案，如果你有一个表，你只需要从外部加入 0 或 1 行，但它可能有多个，你需要订购你的加入表：

var qry = Foos.GroupJoin(
      Bars.OrderByDescending(b => b.Id),
      foo => foo.Foo_Id,
      bar => bar.Foo_Id,
      (f, bs) => new { Foo = f, Bar = bs.FirstOrDefault() });

否则，您在联接中获得的哪一行将是随机的（或者更具体地说，无论哪个数据库碰巧先找到）。

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虽然接受的答案有效并且对 Linq to Objects 有好处，但它让我感到烦恼的是 SQL 查询不仅仅是一个直接的左外连接。

以下代码依赖于 LinqKit Project，它允许您传递表达式并将它们调用到您的查询中。

static IQueryable<TResult> LeftOuterJoin<TSource,TInner, TKey, TResult>(
     this IQueryable<TSource> source, 
     IQueryable<TInner> inner, 
     Expression<Func<TSource,TKey>> sourceKey, 
     Expression<Func<TInner,TKey>> innerKey, 
     Expression<Func<TSource, TInner, TResult>> result
    ) {
    return from a in source.AsExpandable()
            join b in inner on sourceKey.Invoke(a) equals innerKey.Invoke(b) into c
            from d in c.DefaultIfEmpty()
            select result.Invoke(a,d);
}

可以如下使用

Table1.LeftOuterJoin(Table2, x => x.Key1, x => x.Key2, (x,y) => new { x,y});

---

将 Marc Gravell 的答案转换为扩展方法，我做了以下操作。

internal static IEnumerable<Tuple<TLeft, TRight>> LeftJoin<TLeft, TRight, TKey>(
    this IEnumerable<TLeft> left,
    IEnumerable<TRight> right,
    Func<TLeft, TKey> selectKeyLeft,
    Func<TRight, TKey> selectKeyRight,
    TRight defaultRight = default(TRight),
    IEqualityComparer<TKey> cmp = null)
{
    return left.GroupJoin(
            right,
            selectKeyLeft,
            selectKeyRight,
            (x, y) => new Tuple<TLeft, IEnumerable<TRight>>(x, y),
            cmp ?? EqualityComparer<TKey>.Default)
        .SelectMany(
            x => x.Item2.DefaultIfEmpty(defaultRight),
            (x, y) => new Tuple<TLeft, TRight>(x.Item1, y));
}

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Marc Gravell 的回答变成了支持 IQueryable<T> 接口的扩展方法，在 this answer 中给出，并增加了对 C# 8.0 NRT 的支持，如下所示：

#nullable enable
using LinqKit;
using LinqKit.Core;
using System.Linq.Expressions;

...

/// <summary>
/// Left join queryable. Linq to SQL compatible. IMPORTANT: any Includes must be put on the source collections before calling this method.
/// </summary>
public static IQueryable<TResult> LeftJoin<TOuter, TInner, TKey, TResult>(
    this IQueryable<TOuter> outer,
    IQueryable<TInner> inner,
    Expression<Func<TOuter, TKey>> outerKeySelector,
    Expression<Func<TInner, TKey>> innerKeySelector,
    Expression<Func<TOuter, TInner?, TResult>> resultSelector)
{
    return outer
        .AsExpandable()
        .GroupJoin(
            inner,
            outerKeySelector,
            innerKeySelector,
            (outerItem, innerItems) => new { outerItem, innerItems })
        .SelectMany(
            joinResult => joinResult.innerItems.DefaultIfEmpty(),
            (joinResult, innerItem) =>
                resultSelector.Invoke(joinResult.outerItem, innerItem));
}

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