LINQ体验(6)——LINQ to SQL语句之Join和Order By

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

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：

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.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

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
    };

作者：李永京（YJingLee's Blog）

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