[.net]LINQ体验(3)——LINQ语句之Join和Order By(装)
Join操作
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.Customers from o in c.Orders where 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.Products where p.Supplier.Country == "USA" && p.UnitsInStock == 0 select 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.Employees from et in e.EmployeeTerritories where 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.Employees from e2 in e1.Employees where e1.City == e2.City select 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(两个表联合查询)
var q = from c in db.Customers join o in db.Orders on c.CustomerID equals o.CustomerID into orders select 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。如图所示:
![GroupJoin示意图](https://lyj.cnblogs.com/images/cnblogs_com/lyj/LINQ/LINQ16.gif)
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(三个表联合查询)
var q = from c in db.Customers join o in db.Orders on c.CustomerID equals o.CustomerID into ords join e in db.Employees on c.City equals e.City into emps select 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.Employees join o in db.Orders on e equals o.Employee into ords from 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.Customers join o in db.Orders on c.CustomerID equals o.CustomerID into ords let z = c.City + c.Country from o in ords select new {c.ContactName, o.OrderID, z};
5.Composite Key(组合键)
var q = from o in db.Orders from p in db.Products join d in db.OrderDetails on new {o.OrderID, p.ProductID} equals new {d.OrderID, d.ProductID} into details from d in details select 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.Orders join e in db.Employees on o.EmployeeID equals (int?)e.EmployeeID into emps from e in emps select 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.Employees orderby e.HireDate select 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.Orders where o.ShipCity == "London" orderby o.Freight select o;
3.降序排序
var q = from p in db.Products orderby p.UnitPrice descending select 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.Customers orderby c.City, c.ContactName select 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.Orders where o.EmployeeID == 1 orderby o.ShipCountry, o.Freight descending select o;
6.带GroupBy形式
var categories = from p in db.Products group p by p.CategoryID into g orderby g.Key select new { g.Key, MostExpensiveProducts = from p2 in g where p2.UnitPrice == g.Max(p3 => p3.UnitPrice) select p2 };
好了,今天就说到这里,下篇继续介绍