如何在 lambda 中进行 sql 连接？

有时，我会偶然发现这个问题：我使用了 lambda 连接的子集。鉴于我可以使用任何 LINQ 扩展，我应该如何实现以下连接：

为了简单起见，表定义为

CREATE TABLE [dbo].[TableA] (
    [Key]             INT            IDENTITY (1, 1) NOT NULL,
    [Value]           NVARCHAR (MAX) NULL,
    CONSTRAINT [PK_TableA] PRIMARY KEY CLUSTERED ([Key] ASC)
);

CREATE TABLE [dbo].[TableB] (
    [Key]             INT            IDENTITY (1, 1) NOT NULL,
    [Value]           NVARCHAR (MAX) NULL,
    CONSTRAINT [PK_TableB] PRIMARY KEY CLUSTERED ([Key] ASC)
);

或者如果您更喜欢先写代码

public class TableContext : DbContext
{
    public DbSet<B> TableB { get; set; }
    public DbSet<A> TableA { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer(ConnectionString);
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<TableB>().Property(o => o.Key).UseSqlServerIdentityColumn();
        modelBuilder.Entity<TableA>().Property(o => o.Key).UseSqlServerIdentityColumn();
    }
}

public class B : IKeyValue
{
    public int Key { get; set; }
    public string Value { get; set; }
}

public class A : IKeyValue
{
    public int Key { get; set; }
    public string Value { get; set; }
}

public interface IKeyValue
{
    int Key { get; set; }
    string Value { get; set; }
}

作为我的努力

((A 相交不 B) 并集 (A 相交 B)) https://www.wolframalpha.com/input/?i=((A%20intersect%20not%20B)%20union%20(A%20intersect%20B))

var leftOuterJoin = TableA
  .GroupJoin(
    TableB, 
    a => a.Key,
    b => b.Key,
    (x, y) => new { TableA = x, TableA = y })
  .SelectMany(
    x => x.TableB.DefaultIfEmpty(),
    (x, y) => new { TableA = x.TableA, TableB = y});

（A路口B） https://www.wolframalpha.com/input/?i=(A%20intersects%20B)

var innerJoin = TableA
  .Join(
    TableB, 
    a => a.Key,
    b => b.Key,
    (x, y) => x)

（A联盟B） https://www.wolframalpha.com/input/?i=(A%20union%20B)

var fullOuterJoin = TableA
  .FullOuterJoin(
    TableB, 
    a => a.Key, 
    b => b.Key, 
    (x, y, Key) => new {x, y})

对你来说最重要的是知道如何执行INNER JOIN and an OUTER JOIN.

For the 内部联接你用JOIN从 LINQ 像这样：

var result = 
    TableA
    .Join(TableB, left => left.Id, right => right.ForeignKeyToTableA, 
       (left, right) => new { TableAColumns = left, TableBColumns = right });

The 外连接你已经在你的例子中展示了。

现在您需要混合您所知道的内容，以获得所需的结果。

例如执行全外连接在 LINQ 中执行类似以下伪代码的操作：

SELECT TableA.*, TableB.* FROM TableA LEFT OUTER JOIN TableB
UNION
SELECT TableA.*, TableB.* FROM TableB LEFT OUTER JOIN TableA

在 LINQ 中，这将如下所示：

var fullOuterJoin =
            (
                TableA
                .GroupJoin(TableB, 
                    left => left.Id, right => right.ForeignKeyId, 
                    (left, right) => new { TableA = left, TableB = right })
                .SelectMany(p => p.TableB.DefaultIfEmpty(), (x, y) => 
                    new { TableA = x.TableA, TableB = y })
            )
            .Union
            (
                TableB
                .GroupJoin(TableA, 
                    left => left.Id, right => right.ForeignKeyId, 
                    (left, right) => new { TableA = right, TableB = left })
                .SelectMany(p => p.TableA.DefaultIfEmpty(), (x, y) => 
                    new { TableA = y, TableB = x.TableB })
            );

您的图像的最后一个示例将是：

var fullOuterJoinOnlyWithNulls =
            fullOuterJoin
            .Where(p => p.TableA == null || p.TableB == null);

A 右外连接无非是一个左外连接您可以像这样交换结果列：

var rightOuterJoin =
            (
                TableB
                .GroupJoin(TableA,
                    left => left.Id, right => right.ForeignKeyId,
                    (left, right) => new { TableA = right, TableB = left })
                .SelectMany(p => p.TableA.DefaultIfEmpty(), (x, y) =>
                    new { TableA = y, TableB = x.TableB })
            );

像这样，您可以构建所有示例场景。只需检查表格即可null需要的时候。