每个继承类一个表

1、每个继承一个表场景，适用与主表及主表扩展表情景

　　主表与扩展表是1：0...1关系。

2.创建表

CREATE TABLE [Chapter2].[Business](
    [BusinessId] [int] IDENTITY(1,1) primary key,
    [Name] [varchar](50) NOT NULL,
    [LicenseNumber] [varchar](20) NOT NULL
    )
GO

CREATE TABLE [Chapter2].[eCommerce](
    [BusinessId] [int] PRIMARY Key ,
    [URL] [varchar](1024) NULL
)


ALTER TABLE [Chapter2].[eCommerce]  WITH CHECK ADD  CONSTRAINT [FK_eCommerce_Business] FOREIGN KEY([BusinessId])
REFERENCES [Chapter2].[Business] ([BusinessId])
GO

GO


CREATE TABLE [Chapter2].[Retail](
    [BusinessId] [int] NOT NULL,
    [Address] [varchar](50) NOT NULL,
    [City] [varchar](50) NOT NULL,
    [State] [varchar](2) NOT NULL,
    [ZIPCode] [varchar](9) NOT NULL
    )
GO

ALTER TABLE [Chapter2].[Retail]  WITH CHECK ADD  CONSTRAINT [FK_Retail_Business] FOREIGN KEY([BusinessId])
REFERENCES [Chapter2].[Business] ([BusinessId])
GO

3、创建项目，添加EF引用，注意链接字符串

4、添加类　　

    public class BusinessContext : DbContext
    {
        public DbSet<Business> Businesses { get; set; }
        public BusinessContext() : base("EFRecipesEntities")
        {

        }
    }

    [Table("Business", Schema = "Chapter2")]
    public class Business
    {
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int BusinessId { get; set; }
        public string Name { get; set; }
        public string LicenseNumber { get; set; }
    }

    [Table("eCommerce", Schema = "Chapter2")]
    public class eCommerce : Business
    {
        public string URL { get; set; }
    }

    [Table("Retail", Schema = "Chapter2")]
    public class Retail : Business
    {
        public string Address { get; set; }
        public string City { get; set; }
        public string State { get; set; }


        public string ZIPCode { get; set; }

    }

5、修改Main方法代码如下

 static void Main(string[] args)
        {
            using (var context = new BusinessContext())
            {
                var business = new Business() { Name = "Corner Dry Cleaning", LicenseNumber = "100x1" };

                context.Businesses.Add(business);
                var retail = new Retail
                {
                    Name = "Shop and Save",
                    LicenseNumber = "200C",
                    Address = "101 Main",
                    City = "Anytown",
                    State = "TX",
                    ZIPCode = "76106"
                };
                context.Businesses.Add(retail);
                var web = new eCommerce()
                {
                    Name = "BuyNow.com",
                    LicenseNumber = "300AB",
                    URL = "www.buynow.com"
                };
                context.Businesses.Add(web);
                context.SaveChanges();
            }

            using (var context = new BusinessContext())
            {
                Console.WriteLine("
--- All Businesses ---");

                foreach (var b in context.Businesses)
                {
                    Console.WriteLine($"{b.Name}(#{b.LicenseNumber})");
                }

                Console.WriteLine("
 --- Retail Businesses ---");

                foreach (var r in context.Businesses.OfType<Retail>())
                {
                    Console.WriteLine($"{r.Name},(#{r.LicenseNumber})");
                    Console.WriteLine(r.Address);
                    Console.WriteLine($"{r.City},{r.State}{r.ZIPCode}");
                }

                Console.WriteLine("
--- eCommerce Businesses ---");
                foreach (var e in context.Businesses.OfType<eCommerce>())
                {
                    Console.WriteLine("{0} (#{1})", e.Name, e.LicenseNumber);
                    Console.WriteLine("Online address is: {0}", e.URL);
                }

                Console.ReadKey();

            }
        }

6、运行结果

7、运行时使用SQlProfile 监控语句如下

SELECT 
        [Extent2].[BusinessId] AS [BusinessId], 
        [Extent2].[Address] AS [Address], 
        [Extent2].[City] AS [City], 
        [Extent2].[State] AS [State], 
        [Extent2].[ZIPCode] AS [ZIPCode], 
        cast(1 as bit) AS [C1]
        FROM [Chapter2].[Retail] AS [Extent2] 

SELECT 
        [Extent3].[BusinessId] AS [BusinessId], 
        [Extent3].[URL] AS [URL], 
        cast(1 as bit) AS [C1]
        FROM [Chapter2].[eCommerce] AS [Extent3]

SELECT 
    CASE WHEN (( NOT (([Project1].[C1] = 1) AND ([Project1].[C1] IS NOT NULL))) AND ( NOT (([Project2].[C1] = 1) AND ([Project2].[C1] IS NOT NULL)))) THEN '0X' WHEN (([Project2].[C1] = 1) AND ([Project2].[C1] IS NOT NULL)) THEN '0X0X' ELSE '0X1X' END AS [C1], 
    [Extent1].[BusinessId] AS [BusinessId], 
    [Extent1].[Name] AS [Name], 
    [Extent1].[LicenseNumber] AS [LicenseNumber], 
    CASE WHEN (( NOT (([Project1].[C1] = 1) AND ([Project1].[C1] IS NOT NULL))) AND ( NOT (([Project2].[C1] = 1) AND ([Project2].[C1] IS NOT NULL)))) THEN CAST(NULL AS varchar(1)) WHEN (([Project2].[C1] = 1) AND ([Project2].[C1] IS NOT NULL)) THEN [Project2].[URL] END AS [C2], 
    CASE WHEN (( NOT (([Project1].[C1] = 1) AND ([Project1].[C1] IS NOT NULL))) AND ( NOT (([Project2].[C1] = 1) AND ([Project2].[C1] IS NOT NULL)))) THEN CAST(NULL AS varchar(1)) WHEN (([Project2].[C1] = 1) AND ([Project2].[C1] IS NOT NULL)) THEN CAST(NULL AS varchar(1)) ELSE [Project1].[Address] END AS [C3], 
    CASE WHEN (( NOT (([Project1].[C1] = 1) AND ([Project1].[C1] IS NOT NULL))) AND ( NOT (([Project2].[C1] = 1) AND ([Project2].[C1] IS NOT NULL)))) THEN CAST(NULL AS varchar(1)) WHEN (([Project2].[C1] = 1) AND ([Project2].[C1] IS NOT NULL)) THEN CAST(NULL AS varchar(1)) ELSE [Project1].[City] END AS [C4], 
    CASE WHEN (( NOT (([Project1].[C1] = 1) AND ([Project1].[C1] IS NOT NULL))) AND ( NOT (([Project2].[C1] = 1) AND ([Project2].[C1] IS NOT NULL)))) THEN CAST(NULL AS varchar(1)) WHEN (([Project2].[C1] = 1) AND ([Project2].[C1] IS NOT NULL)) THEN CAST(NULL AS varchar(1)) ELSE [Project1].[State] END AS [C5], 
    CASE WHEN (( NOT (([Project1].[C1] = 1) AND ([Project1].[C1] IS NOT NULL))) AND ( NOT (([Project2].[C1] = 1) AND ([Project2].[C1] IS NOT NULL)))) THEN CAST(NULL AS varchar(1)) WHEN (([Project2].[C1] = 1) AND ([Project2].[C1] IS NOT NULL)) THEN 
CAST(NULL AS varchar(1)) ELSE [Project1].[ZIPCode] END AS [C6]
    FROM   [Chapter2].[Business] AS [Extent1]
    LEFT OUTER JOIN  (SELECT 
        [Extent2].[BusinessId] AS [BusinessId], 
        [Extent2].[Address] AS [Address], 
        [Extent2].[City] AS [City], 
        [Extent2].[State] AS [State], 
        [Extent2].[ZIPCode] AS [ZIPCode], 
        cast(1 as bit) AS [C1]
        FROM [Chapter2].[Retail] AS [Extent2] ) AS [Project1] ON [Extent1].[BusinessId] = [Project1].[BusinessId]
    LEFT OUTER JOIN  (SELECT 
        [Extent3].[BusinessId] AS [BusinessId], 
        [Extent3].[URL] AS [URL], 
        cast(1 as bit) AS [C1]
        FROM [Chapter2].[eCommerce] AS [Extent3] ) AS [Project2] ON [Extent1].[BusinessId] = [Project2].[BusinessId]

总结：EF 查询时为扩展表添加bit类型字段，存在时为1，根据是否存在该字段来确定扩展表信息是否存在。

　　通过Profile监控看到该方法使用了临时表查询，数据量大时会存在性能问题。

　　可借鉴方法临时表中添加bit字段，通过bit字段是否为null，来决定是否操作此记录。