LINQ to SQL语句(25)之继承
系列文章导航：
LINQ to SQL语句(2)之Select/Distinct
LINQ to SQL语句(3)之Count/Sum/Min/Max/Avg
LINQ to SQL语句(6)之Group By/Having
LINQ to SQL语句(7)之Exists/In/Any/All/Contains
LINQ to SQL语句(8)之Concat/Union/Intersect/Except
LINQ to SQL语句(9)之Top/Bottom和Paging和SqlMethods
LINQ to SQL语句(12)之Delete和使用Attach
LINQ to SQL语句(14)之Null语义和DateTime
LINQ to SQL语句(19)之ADO.NET与LINQ to SQL
1.一般形式
日常我们经常写的形式，对单表查询。
var cons = from c in db.Contacts select c; foreach (var con in cons) { Console.WriteLine("Company name: {0}", con.CompanyName); Console.WriteLine("Phone: {0}", con.Phone); Console.WriteLine("This is a {0}", con.GetType()); }
2.OfType形式
这里我仅仅让其返回顾客的联系方式。
var cons = from c in db.Contacts.OfType<CustomerContact>() select c;
初步学习，我们还是看看生成的SQL语句，这样容易理解。在SQL语句中查询了ContactType为Customer的联系方式。
SELECT [t0].[ContactType], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address],[t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Fax],[t0].[ContactID], [t0].[CompanyName], [t0].[Phone] FROM [dbo].[Contacts] AS [t0] WHERE ([t0].[ContactType] = @p0) AND ([t0].[ContactType] IS NOT NULL) -- @p0: Input NVarChar (Size = 8; Prec = 0; Scale = 0) [Customer]
3.IS形式
这个例子查找一下发货人的联系方式。
var cons = from c in db.Contacts where c is ShipperContact select c;
生成的SQL语句如下：查询了ContactType为Shipper的联系方式。大致一看好像很上面的一样，其实这里查询出来的列多了很多。实际上是Contacts表的全部字段。
SELECT [t0].[ContactType], [t0].[ContactID], [t0].[CompanyName], [t0].[Phone],[t0].[HomePage], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Fax],[t0].[PhotoPath], [t0].[Photo], [t0].[Extension] FROM [dbo].[Contacts] AS [t0] WHERE ([t0].[ContactType] = @p0) AND ([t0].[ContactType] IS NOT NULL) -- @p0: Input NVarChar (Size = 7; Prec = 0; Scale = 0) [Shipper]
4.AS形式
这个例子就通吃了，全部查找了一番。
var cons = from c in db.Contacts select c as FullContact;
生成SQL语句如下：查询整个Contacts表。
SELECT [t0].[ContactType], [t0].[HomePage], [t0].[ContactName], [t0].[ContactTitle],[t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Fax], [t0].[ContactID], [t0].[CompanyName], [t0].[Phone], [t0].[PhotoPath],[t0].[Photo], [t0].[Extension] FROM [dbo].[Contacts] AS [t0]
5.Cast形式
使用Case形式查找出在伦敦的顾客的联系方式。
var cons = from c in db.Contacts where c.ContactType == "Customer" && ((CustomerContact)c).City == "London" select c;
生成SQL语句如下，自己可以看懂了。
SELECT [t0].[ContactType], [t0].[ContactID], [t0].[CompanyName], [t0].[Phone], [t0].[HomePage],[t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Fax], [t0].[PhotoPath], [t0].[Photo], [t0].[Extension]FROM [dbo].[Contacts] AS [t0] WHERE ([t0].[ContactType] = @p0) AND ([t0].[City] = @p1) -- @p0: Input NVarChar (Size = 8; Prec = 0; Scale = 0) [Customer] -- @p1: Input NVarChar (Size = 6; Prec = 0; Scale = 0) [London]
6.UseAsDefault形式
当插入一条记录时，使用默认的映射关系了，但是在查询时，使用继承的关系了。具体看看生成的SQL语句就直截了当了。
//插入一条数据默认使用正常的映射关系 Contact contact = new Contact() { ContactType = null, CompanyName = "Unknown Company", Phone = "333-444-5555" }; db.Contacts.InsertOnSubmit(contact); db.SubmitChanges(); //查询一条数据默认使用继承映射关系 var con = (from c in db.Contacts where c.CompanyName == "Unknown Company" && c.Phone == "333-444-5555" select c).First();
生成SQL语句如下：
INSERT INTO [dbo].[Contacts]([ContactType], [CompanyName], [Phone]) VALUES (@p0, @p1, @p2) SELECT TOP (1) [t0].[ContactType], [t0].[ContactID], [t0].[CompanyName], [t0].[Phone],[t0].[HomePage], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City],[t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Fax], [t0].[PhotoPath],[t0].[Photo], [t0].[Extension] FROM [dbo].[Contacts] AS [t0] WHERE ([t0].[CompanyName] = @p0) AND ([t0].[Phone] = @p1) -- @p0: Input NVarChar (Size = 15; Prec = 0; Scale = 0) [Unknown Company] -- @p1: Input NVarChar (Size = 12; Prec = 0; Scale = 0) [333-444-5555]
7.插入新的记录
这个例子说明如何插入发货人的联系方式的一条记录。
//1.在插入之前查询一下，没有数据 var ShipperContacts = from sc in db.Contacts.OfType<ShipperContact>() where sc.CompanyName == "Northwind Shipper" select sc; //2.插入数据 ShipperContact nsc = new ShipperContact() { CompanyName = "Northwind Shipper", Phone = "(123)-456-7890" }; db.Contacts.InsertOnSubmit(nsc); db.SubmitChanges(); //3.查询数据，有一条记录 ShipperContacts = from sc in db.Contacts.OfType<ShipperContact>() where sc.CompanyName == "Northwind Shipper" select sc; //4.删除记录 db.Contacts.DeleteOnSubmit(nsc); db.SubmitChanges();
生成SQL语句如下：
SELECT COUNT(*) AS [value] FROM [dbo].[Contacts] AS [t0] WHERE ([t0].[CompanyName] = @p0) AND ([t0].[ContactType] = @p1) AND ([t0].[ContactType] IS NOT NULL) -- @p0: Input NVarChar [Northwind Shipper] -- @p1: Input NVarChar [Shipper] INSERT INTO [dbo].[Contacts]([ContactType], [CompanyName], [Phone]) VALUES (@p0, @p1, @p2) -- @p0: Input NVarChar [Shipper] -- @p1: Input NVarChar [Northwind Shipper] -- @p2: Input NVarChar [(123)-456-7890] SELECT COUNT(*) AS [value] FROM [dbo].[Contacts] AS [t0] WHERE ([t0].[CompanyName] = @p0) AND ([t0].[ContactType] = @p1) AND ([t0].[ContactType] IS NOT NULL) -- @p0: Input NVarChar [Northwind Shipper] -- @p1: Input NVarChar [Shipper] DELETE FROM [dbo].[Contacts] WHERE ([ContactID] = @p0) AND ([ContactType] = @p1) AND ([CompanyName] = @p2) AND ([Phone] = @p3) -- @p0: Input Int [159] -- @p1: Input NVarChar [Shipper] -- @p2: Input NVarChar [Northwind Shipper] -- @p3: Input NVarChar [(123)-456-7890] -- @p4: Input NVarChar [Unknown] -- @p5: Input NVarChar (Size = 8; Prec = 0; Scale = 0) [Supplier] -- @p6: Input NVarChar (Size = 7; Prec = 0; Scale = 0) [Shipper] -- @p7: Input NVarChar (Size = 8; Prec = 0; Scale = 0) [Employee] -- @p8: Input NVarChar (Size = 8; Prec = 0; Scale = 0) [Customer]
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