实体框架左联接

Answers:

135

改编自MSDN，如何使用EF 4退出联接

var query = from u in usergroups
            join p in UsergroupPrices on u.UsergroupID equals p.UsergroupID into gj
            from x in gj.DefaultIfEmpty()
            select new { 
                UsergroupID = u.UsergroupID,
                UsergroupName = u.UsergroupName,
                Price = (x == null ? String.Empty : x.Price) 
            };
— 梅纳姆
source
2
我比结尾处的gj.DefaultIfEmpty（）更好，因为我可以在where或select中使用x！
— 2013年
1
您能解释一下“来自gj.DefaultIfEmpty（）中的x”行吗？
— Alex Dresko 2014年
@AlexDresko这一部分从join中获取所有结果，对于那些没有右手值的结果，将为您提供null（对象的默认值为null）。hth
— Menahem
2
如果有两个以上的表怎么办？
— MohammadHossein R
1
这与efcore略有不同。from x in gj.DefaultIfEmpty()成为from p in gj.DefaultIfEmpty()。docs.microsoft.com/en-us/ef/core/querying/...
— carlin.scott
30

可能有点过大，但是我写了一个扩展方法，因此可以LeftJoin使用Join语法（至少在方法调用表示法中）：

persons.LeftJoin(
    phoneNumbers,
    person => person.Id,
    phoneNumber => phoneNumber.PersonId,
    (person, phoneNumber) => new
        {
            Person = person,
            PhoneNumber = phoneNumber?.Number
        }
);

我的代码只不过GroupJoin是SelectMany向当前表达式树添加和调用而已。但是，它看起来非常复杂，因为我必须自己构建表达式并修改用户在resultSelector参数中指定的表达式树，以使整个树可被LINQ-to-Entities进行翻译。

public static class LeftJoinExtension
{
    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)
    {
        MethodInfo groupJoin = typeof (Queryable).GetMethods()
                                                 .Single(m => m.ToString() == "System.Linq.IQueryable`1[TResult] GroupJoin[TOuter,TInner,TKey,TResult](System.Linq.IQueryable`1[TOuter], System.Collections.Generic.IEnumerable`1[TInner], System.Linq.Expressions.Expression`1[System.Func`2[TOuter,TKey]], System.Linq.Expressions.Expression`1[System.Func`2[TInner,TKey]], System.Linq.Expressions.Expression`1[System.Func`3[TOuter,System.Collections.Generic.IEnumerable`1[TInner],TResult]])")
                                                 .MakeGenericMethod(typeof (TOuter), typeof (TInner), typeof (TKey), typeof (LeftJoinIntermediate<TOuter, TInner>));
        MethodInfo selectMany = typeof (Queryable).GetMethods()
                                                  .Single(m => m.ToString() == "System.Linq.IQueryable`1[TResult] SelectMany[TSource,TCollection,TResult](System.Linq.IQueryable`1[TSource], System.Linq.Expressions.Expression`1[System.Func`2[TSource,System.Collections.Generic.IEnumerable`1[TCollection]]], System.Linq.Expressions.Expression`1[System.Func`3[TSource,TCollection,TResult]])")
                                                  .MakeGenericMethod(typeof (LeftJoinIntermediate<TOuter, TInner>), typeof (TInner), typeof (TResult));

        var groupJoinResultSelector = (Expression<Func<TOuter, IEnumerable<TInner>, LeftJoinIntermediate<TOuter, TInner>>>)
                                      ((oneOuter, manyInners) => new LeftJoinIntermediate<TOuter, TInner> {OneOuter = oneOuter, ManyInners = manyInners});

        MethodCallExpression exprGroupJoin = Expression.Call(groupJoin, outer.Expression, inner.Expression, outerKeySelector, innerKeySelector, groupJoinResultSelector);

        var selectManyCollectionSelector = (Expression<Func<LeftJoinIntermediate<TOuter, TInner>, IEnumerable<TInner>>>)
                                           (t => t.ManyInners.DefaultIfEmpty());

        ParameterExpression paramUser = resultSelector.Parameters.First();

        ParameterExpression paramNew = Expression.Parameter(typeof (LeftJoinIntermediate<TOuter, TInner>), "t");
        MemberExpression propExpr = Expression.Property(paramNew, "OneOuter");

        LambdaExpression selectManyResultSelector = Expression.Lambda(new Replacer(paramUser, propExpr).Visit(resultSelector.Body), paramNew, resultSelector.Parameters.Skip(1).First());

        MethodCallExpression exprSelectMany = Expression.Call(selectMany, exprGroupJoin, selectManyCollectionSelector, selectManyResultSelector);

        return outer.Provider.CreateQuery<TResult>(exprSelectMany);
    }

    private class LeftJoinIntermediate<TOuter, TInner>
    {
        public TOuter OneOuter { get; set; }
        public IEnumerable<TInner> ManyInners { get; set; }
    }

    private class Replacer : ExpressionVisitor
    {
        private readonly ParameterExpression _oldParam;
        private readonly Expression _replacement;

        public Replacer(ParameterExpression oldParam, Expression replacement)
        {
            _oldParam = oldParam;
            _replacement = replacement;
        }

        public override Expression Visit(Expression exp)
        {
            if (exp == _oldParam)
            {
                return _replacement;
            }

            return base.Visit(exp);
        }
    }
}
— 费罗
source
2
感谢您的扩展费力。
— Fergers '16
这仍然很棒。谢谢！
— TheGeekYouNeed
1
在.NET Framework 4.6.2中进行了测试，它可以按预期工作（即，生成LEFT OUTER JOIN）。我想知道它是否适用于.NET Core。谢谢。
— 阿列克谢
23

请让您的生活更轻松（不要使用加入组）：

var query = from ug in UserGroups
            from ugp in UserGroupPrices.Where(x => x.UserGroupId == ug.Id).DefaultIfEmpty()
            select new 
            { 
                UserGroupID = ug.UserGroupID,
                UserGroupName = ug.UserGroupName,
                Price = ugp != null ? ugp.Price : 0 //this is to handle nulls as even when Price is non-nullable prop it may come as null from SQL (result of Left Outer Join)
            };
— 托马斯·斯科莫拉（Tomasz Skomra）
source
2
避免加入团队是一个观点问题，但这当然是一个有效的观点。Price = ugp.Price如果Price是不可为空的属性，则可能失败，并且左联接未给出任何结果。
1
同意上面的观点，但是有两个以上的表，这种方法非常易于阅读和维护。
— Tomasz Skomra
1
我们可以检查是否ugp == NULL设置了默认值Price。
— Hp93
完美：)
— MohammadHossein R
1
太棒了！我更喜欢此解决方案以提高可读性。而且，这使更多的联接（即从3个或更多表）变得更加容易！我成功地将它用于2个左联接（即3个表）。
— 杰里米·莫伦
4

如果您更喜欢方法调用表示法，则可以SelectMany结合使用来强制进行左联接DefaultIfEmpty。至少在Entity Framework 6上达到了SQL Server。例如：

using(var ctx = new MyDatabaseContext())
{
    var data = ctx
    .MyTable1
    .SelectMany(a => ctx.MyTable2
      .Where(b => b.Id2 == a.Id1)
      .DefaultIfEmpty()
      .Select(b => new
      {
        a.Id1,
        a.Col1,
        Col2 = b == null ? (int?) null : b.Col2,
      }));
}

（请注意，这MyTable2.Col2是类型的列int）。生成的SQL将如下所示：

SELECT 
    [Extent1].[Id1] AS [Id1], 
    [Extent1].[Col1] AS [Col1], 
    CASE WHEN ([Extent2].[Col2] IS NULL) THEN CAST(NULL AS int) ELSE  CAST( [Extent2].[Col2] AS int) END AS [Col2]
    FROM  [dbo].[MyTable1] AS [Extent1]
    LEFT OUTER JOIN [dbo].[MyTable2] AS [Extent2] ON [Extent2].[Id2] = [Extent1].[Id1]
— 地亚哥
source
对我来说，这会生成一些带有“ CROSS APPLY”的极其缓慢的查询。
— Meekohi
2

对于2个或更多左联接（左联接creatorUser和InitiatorUser）

IQueryable<CreateRequestModel> queryResult = from r in authContext.Requests
                                             join candidateUser in authContext.AuthUsers
                                             on r.CandidateId equals candidateUser.Id
                                             join creatorUser in authContext.AuthUsers
                                             on r.CreatorId equals creatorUser.Id into gj
                                             from x in gj.DefaultIfEmpty()
                                             join initiatorUser in authContext.AuthUsers
                                             on r.InitiatorId equals initiatorUser.Id into init
                                             from x1 in init.DefaultIfEmpty()

                                             where candidateUser.UserName.Equals(candidateUsername)
                                             select new CreateRequestModel
                                             {
                                                 UserName = candidateUser.UserName,
                                                 CreatorId = (x == null ? String.Empty : x.UserName),
                                                 InitiatorId = (x1 == null ? String.Empty : x1.UserName),
                                                 CandidateId = candidateUser.UserName
                                             };
— 德米特里·马图宁（Dmitrii Matunin）
source
1

通过在主模型上调用DefaultIfEmpty（），我能够做到这一点。这使我可以在延迟加载的实体上保留联接，对我而言似乎更易读：

        var complaints = db.Complaints.DefaultIfEmpty()
            .Where(x => x.DateStage1Complete == null || x.DateStage2Complete == null)
            .OrderBy(x => x.DateEntered)
            .Select(x => new
            {
                ComplaintID = x.ComplaintID,
                CustomerName = x.Customer.Name,
                CustomerAddress = x.Customer.Address,
                MemberName = x.Member != null ? x.Member.Name: string.Empty,
                AllocationName = x.Allocation != null ? x.Allocation.Name: string.Empty,
                CategoryName = x.Category != null ? x.Category.Ssl_Name : string.Empty,
                Stage1Start = x.Stage1StartDate,
                Stage1Expiry = x.Stage1_ExpiryDate,
                Stage2Start = x.Stage2StartDate,
                Stage2Expiry = x.Stage2_ExpiryDate
            });
— 威廉·罗宾逊
source
1
在这里，您根本不需要.DefaultIfEmpty()：它仅影响在db.Complains为空时发生的情况。db.Complains.Where(...).OrderBy(...).Select(x => new { ..., MemberName = x.Member != null ? x.Member.Name : string.Empty, ... })（没有任何参数.DefaultIfEmpty()）将已经执行左联接（假设该Member属性被标记为可选）。
1

如果UserGroups与UserGroupPrices表具有一对多关系，则在EF中，一旦在代码中定义了该关系，例如：

//In UserGroups Model
public List<UserGroupPrices> UserGrpPriceList {get;set;}

//In UserGroupPrices model
public UserGroups UserGrps {get;set;}

您可以通过以下简单方法拉出左联接结果集：

var list = db.UserGroupDbSet.ToList();

假定左表的DbSet是UserGroupDbSet，其中将包括UserGrpPriceList，该表是右表中所有关联记录的列表。

— Jaggan_j
source
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