如何在lambda中进行sql连接？

Question

我不时地偶然发现这个问题,我使用了lambda连接的一个子集.鉴于我可以使用任何LINQ扩展,我应该如何实现以下连接:

为简单起见,表格定义为

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)
);

或者如果你首先喜欢代码

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交叉而非B)联合(A交叉B))

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与B相交)

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

(工会B)

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

Answer 1

对你来说最重要的是知道如何表演INNER JOIN和表演OUTER JOIN.

对于INNER JOIN,您可以使用JOINLINQ,如下所示:

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

您已在示例中显示的OUTER JOIN.

现在你需要混合你所知道的,以获得理想的结果.

例如,要执行FULL OUTER JOIN,请在LINQ中执行类似此伪代码的操作:

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

这将在LINQ中如下:

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 })
            );

那么你的图像的最后一个例子是:

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

一个RIGHT OUTER JOIN不过是一个LEFT OUTER JOIN,你换你的结果列如下:

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.只需在需要时检查表是否为null.