LINQ to SQL语句之Join和Order By
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原文地址:http://www.cnblogs.com/yibinboy/articles/1305917.html
适用场景:在我们表关系中有一对一关系,一对多关系,多对多关系等。对各个表之间的关系,就用这些实现对多个表的操作。
说明:在Join操作中,分别为Join(Join查询), SelectMany(Select一对多选择)和GroupJoin(分组Join查询)。
该扩展方法对两个序列中键匹配的元素进行inner join操作
SelectMany
说明:我们在写查询语句时,如果被翻译成SelectMany需要满足2个条件。1:查询语句中没有join和into,2:必须出现EntitySet。在我们表关系中有一对一关系,一对多关系,多对多关系等,下面分别介绍一下。
1.1 to Many关系:
var q =from c in db.Customersfrom o in c.Orderswhere c.City == "London"select o;
语句描述:Customers与Orders是一对多关系。即Orders在Customers类中以EntitySet形式出现。所以第二个from是从c.Orders而不是db.Orders里进行筛选。
var q =from p in db.Productswhere p.Supplier.Country == "USA" && p.UnitsInStock == 0select p;
语句描述:这一句使用了p.Supplier.Country条件,间接关联了Supplier表。生成SQL语句为:
SELECT [t0].[ProductID], [t0].[ProductName], [t0].[SupplierID],[t0].[CategoryID],[t0].[QuantityPerUnit],[t0].[UnitPrice],[t0].[UnitsInStock], [t0].[UnitsOnOrder],[t0].[ReorderLevel],[t0].[Discontinued] FROM [dbo].[Products] AS [t0]LEFT OUTER JOIN [dbo].[Suppliers] AS [t1] ON[t1].[SupplierID] = [t0].[SupplierID]WHERE ([t1].[Country] = @p0) AND ([t0].[UnitsInStock] = @p1)-- @p0: Input NVarChar (Size = 3; Prec = 0; Scale = 0) [USA]-- @p1: Input Int (Size = 0; Prec = 0; Scale = 0) [0]
2.Many to Many关系:
var q =from e in db.Employeesfrom et in e.EmployeeTerritorieswhere e.City == "Seattle"select new{e.FirstName,e.LastName,et.Territory.TerritoryDescription};
说明:多对多关系一般会涉及三个表(如果有一个表是自关联的,那有可能只有2个表)。这一句语句涉及Employees, EmployeeTerritories, Territories三个表。它们的关系是1:M:1。Employees和Territories没有很明确的关系。
语句描述:这条生成SQL语句为:
SELECT [t0].[FirstName], [t0].[LastName], [t2].[TerritoryDescription]FROM [dbo].[Employees] AS [t0] CROSS JOIN [dbo].[EmployeeTerritories]AS [t1] INNER JOIN [dbo].[Territories] AS [t2] ON[t2].[TerritoryID] = [t1].[TerritoryID]WHERE ([t0].[City] = @p0) AND ([t1].[EmployeeID] = [t0].[EmployeeID])-- @p0: Input NVarChar (Size = 7; Prec = 0; Scale = 0) [Seattle]
3.自关联关系:
var q =from e1 in db.Employeesfrom e2 in e1.Employeeswhere e1.City == e2.Cityselect new {FirstName1 = e1.FirstName, LastName1 = e1.LastName,FirstName2 = e2.FirstName, LastName2 = e2.LastName,e1.City};
生成SQL语句为:
SELECT [t0].[FirstName] AS [FirstName1], [t0].[LastName] AS [LastName1],[t1].[FirstName] AS [FirstName2], [t1].[LastName] AS [LastName2],[t0].[City] FROM [dbo].[Employees] AS [t0],[dbo].[Employees] AS [t1] WHERE ([t0].[City] = [t1].[City]) AND ([t1].[ReportsTo] = [t0].[EmployeeID])
GroupJoin
像上面所说的,没有join和into,被翻译成SelectMany,同时有join和into时,那么就被翻译为GroupJoin。在这里into的概念是对其结果进行重新命名。
1.Two way join
var q =from c in db.Customersjoin o in db.Orders on c.CustomerIDequals o.CustomerID into ordersselect new{c.ContactName,OrderCount = orders.Count()};
说明:在一对多关系中,左边是1,它每条记录为c(from c in db.Customers),右边是Many,其每条记录叫做o ( join o in db.Orders ),每对应左边的一个c,就会有一组o,那这一组o,就叫做orders,也就是说,我们把一组o命名为orders,这就是into用途。这也就是为什么在select语句中,orders可以调用聚合函数Count。在T-SQL中,使用其内嵌的T-SQL返回值作为字段值。
生成SQL语句为:
SELECT [t0].[ContactName], (SELECT COUNT(*)FROM [dbo].[Orders] AS [t1]WHERE [t0].[CustomerID] = [t1].[CustomerID]) AS [OrderCount]FROM [dbo].[Customers] AS [t0]
2.There way join
var q =from c in db.Customersjoin o in db.Orders on c.CustomerIDequals o.CustomerID into ordsjoin e in db.Employees on c.Cityequals e.City into empsselect new{c.ContactName,ords = ords.Count(),emps = emps.Count()};
生成SQL语句为:
SELECT [t0].[ContactName], (SELECT COUNT(*)FROM [dbo].[Orders] AS [t1]WHERE [t0].[CustomerID] = [t1].[CustomerID]) AS [ords], (SELECT COUNT(*)FROM [dbo].[Employees] AS [t2]WHERE [t0].[City] = [t2].[City]) AS [emps]FROM [dbo].[Customers] AS [t0]
3.Left Outer Join
var q =from e in db.Employeesjoin o in db.Orders on e equals o.Employee into ordsfrom o in ords.DefaultIfEmpty()select new{e.FirstName,e.LastName,Order = o};
说明:以Employees左表,Orders右表,Orders 表中为空时,用null值填充。Join的结果重命名ords,使用DefaultIfEmpty()函数对其再次查询。其最后的结果中有个Order,因为from o in ords.DefaultIfEmpty() 是对ords组再一次遍历,所以,最后结果中的Order并不是一个集合。但是,如果没有from o in ords.DefaultIfEmpty() 这句,最后的select语句写成select new { e.FirstName, e.LastName, Order = ords }的话,那么Order就是一个集合。
4.Projected let assignment
说明:let语句是重命名。let位于第一个from和select语句之间。
var q =from c in db.Customersjoin o in db.Orders on c.CustomerIDequals o.CustomerID into ordslet z = c.City + c.Countryfrom o in ordsselect new{c.ContactName,o.OrderID,z};
5.Composite Key
var q =from o in db.Ordersfrom p in db.Productsjoin d in db.OrderDetailson new{o.OrderID,p.ProductID} equalsnew{d.OrderID,d.ProductID}into detailsfrom d in detailsselect new{o.OrderID,p.ProductID,d.UnitPrice};
说明:使用三个表,并且用匿名类来表示它们之间的关系。它们之间的关系不能用一个键描述清楚,所以用匿名类,来表示组合键。还有一种是两个表之间是用组合键表示关系的,不需要使用匿名类。
6.Nullable/Nonnullable Key Relationship
var q =from o in db.Ordersjoin e in db.Employeeson o.EmployeeID equals(int?)e.EmployeeID into empsfrom e in empsselect new{o.OrderID,e.FirstName};
Order By操作
适用场景:对查询出的语句进行排序,比如按时间排序等等。
说明:按指定表达式对集合排序;延迟,默认是升序,加上descending表示降序,对应的扩展方法是OrderBy和OrderByDescending
1.简单形式
说明:默认为升序
var q =from e in db.Employeesorderby e.HireDateselect e;
2.带条件形式
注意:Where和Order By的顺序并不重要。而在T-SQL中,Where和Order By有严格的位置限制。
var q =from o in db.Orderswhere o.ShipCity == "London"orderby o.Freightselect o;
3.降序排序
var q =from p in db.Productsorderby p.UnitPrice descendingselect p;
4.ThenBy
说明:按多个表达式进行排序,例如先按City排序,当City相同时,按ContactName排序。
var q =from c in db.Customersorderby c.City, c.ContactNameselect c;
说明:这一句用Lambda表达式像这样写:
var q = db.Customers.OrderBy(c => c.City).ThenBy(c => c.ContactName).ToList();
在T-SQL中没有ThenBy语句,其依然翻译为OrderBy,所以也可以用下面语句来表达:
var q = db.Customers.OrderBy(c => c.ContactName).OrderBy(c => c.City).ToList();
所要注意的是,多个OrderBy操作时,级连方式是按逆序。对于降序的,用相应的降序操作符替换即可。
var q = db.Customers.OrderByDescending(c => c.City).ThenByDescending(c => c.ContactName).ToList();
需要说明的是,OrderBy操作,不支持按type排序,也不支持匿名类。比如
var q = db.Customers.OrderBy(c => new{c.City,c.ContactName}).ToList();
会被抛出异常。错误是前面的操作有匿名类,再跟OrderBy时,比较的是类别。比如
var q = db.Customers.Select(c => new{c.City,c.Address}).OrderBy(c => c).ToList();
如果你想使用OrderBy(c => c),其前提条件是,前面步骤中,所产生的对象的类别必须为C#语言的基本类型。比如下句,这里City为string类型。
var q = db.Customers.Select(c => c.City).OrderBy(c => c).ToList();
5.ThenByDescending
这两个扩展方式都是用在OrderBy/OrderByDescending之后的,第一个ThenBy/ThenByDescending扩展方法作为第二位排序依据,第二个ThenBy/ThenByDescending则作为第三位排序依据,以此类推
var q =from o in db.Orderswhere o.EmployeeID == 1orderby o.ShipCountry, o.Freight descendingselect o;
6.带GroupBy形式
var categories =from p in db.Productsgroup p by p.CategoryID into gorderby g.Keyselect new {g.Key,MostExpensiveProducts =from p2 in gwhere p2.UnitPrice == g.Max(p3 => p3.UnitPrice)select p2};
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