再接再厉VS 2008 sp1 + .NET 3.5 sp1(3) - Entity Framework(实体框架)之详解 Linq To Entities 之二

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再接再厉VS 2008 sp1 + .NET 3.5 sp1(3) - Entity Framework(实体框架)之详解 Linq To Entities 之二

作者： webabcd


介绍
以Northwind为示例数据库，ADO.NET Entity Framework之Linq To Entities

• Select - 选择需要返回的字段
• Where - 筛选
• OrderBy - 正序排序
• OrderByDescending - 倒序排序
• ThenBy - 在 OrderBy 或 OrderByDescending 的基础上再正序排序
• ThenByDescending - 在 OrderBy 或 OrderByDescending 的基础上再倒序排序
• GroupBy - 分组
• Join - 连接
• GroupJoin - 连接后分组

示例
Select

using  (var ctx  =   new  NorthwindEntities())
{
    // Select 对应的 Linq 方法
    var p1 = ctx.Products.Select(p => new { ProductName = "产品名称：" + p.ProductName });
    p1.ToList();

    // Select 对应的查询语法
    var p2 = from p in ctx.Products select new { ProductName = "产品名称：" + p.ProductName };
    p2.ToList();
}

-- Select 对应的 sql 语句

SELECT  
1   AS   [ C1 ] , 
N ' 产品名称： '   +   [ Extent1 ] . [ ProductName ]   AS   [ C2 ]
FROM   [ dbo ] . [ Products ]   AS   [ Extent1 ]

Where

using  (var ctx  =   new  NorthwindEntities())
{
    // Where 对应的 Linq 方法
    var p1 = ctx.Products.Where(p => p.ProductID > 3);
    p1.ToList();

    // Where 对应的查询语法
    var p2 = from p in ctx.Products where p.ProductID > 3 select p;
    p2.ToList();
}

-- Where 对应的 sql 语句

SELECT  
1   AS   [ C1 ] , 
[ Extent1 ] . [ Discontinued ]   AS   [ Discontinued ] , 
[ Extent1 ] . [ ProductID ]   AS   [ ProductID ] , 
[ Extent1 ] . [ ProductName ]   AS   [ ProductName ] , 
[ Extent1 ] . [ QuantityPerUnit ]   AS   [ QuantityPerUnit ] , 
[ Extent1 ] . [ ReorderLevel ]   AS   [ ReorderLevel ] , 
[ Extent1 ] . [ UnitPrice ]   AS   [ UnitPrice ] , 
[ Extent1 ] . [ UnitsInStock ]   AS   [ UnitsInStock ] , 
[ Extent1 ] . [ UnitsOnOrder ]   AS   [ UnitsOnOrder ] , 
[ Extent1 ] . [ CategoryID ]   AS   [ CategoryID ] , 
[ Extent1 ] . [ SupplierID ]   AS   [ SupplierID ]
FROM   [ dbo ] . [ Products ]   AS   [ Extent1 ]
WHERE   [ Extent1 ] . [ ProductID ]   >   3

OrderBy

using  (var ctx  =   new  NorthwindEntities())
{
    // OrderBy 对应的 Linq 方法
    var p1 = ctx.Products.OrderBy(p => p.UnitPrice);
    p1.ToList();

    // OrderBy 对应的查询语法
    var p2 = from p in ctx.Products orderby p.UnitPrice select p;
    p2.ToList();
}

-- OrderBy 对应的 sql 语句

SELECT  
[ Project1 ] . [ C1 ]   AS   [ C1 ] , 
[ Project1 ] . [ Discontinued ]   AS   [ Discontinued ] , 
[ Project1 ] . [ ProductID ]   AS   [ ProductID ] , 
[ Project1 ] . [ ProductName ]   AS   [ ProductName ] , 
[ Project1 ] . [ QuantityPerUnit ]   AS   [ QuantityPerUnit ] , 
[ Project1 ] . [ ReorderLevel ]   AS   [ ReorderLevel ] , 
[ Project1 ] . [ UnitPrice ]   AS   [ UnitPrice ] , 
[ Project1 ] . [ UnitsInStock ]   AS   [ UnitsInStock ] , 
[ Project1 ] . [ UnitsOnOrder ]   AS   [ UnitsOnOrder ] , 
[ Project1 ] . [ CategoryID ]   AS   [ CategoryID ] , 
[ Project1 ] . [ SupplierID ]   AS   [ SupplierID ]
FROM  (  SELECT  
     [ Extent1 ] . [ CategoryID ]   AS   [ CategoryID ] , 
     [ Extent1 ] . [ Discontinued ]   AS   [ Discontinued ] , 
     [ Extent1 ] . [ ProductID ]   AS   [ ProductID ] , 
     [ Extent1 ] . [ ProductName ]   AS   [ ProductName ] , 
     [ Extent1 ] . [ QuantityPerUnit ]   AS   [ QuantityPerUnit ] , 
     [ Extent1 ] . [ ReorderLevel ]   AS   [ ReorderLevel ] , 
     [ Extent1 ] . [ SupplierID ]   AS   [ SupplierID ] , 
     [ Extent1 ] . [ UnitPrice ]   AS   [ UnitPrice ] , 
     [ Extent1 ] . [ UnitsInStock ]   AS   [ UnitsInStock ] , 
     [ Extent1 ] . [ UnitsOnOrder ]   AS   [ UnitsOnOrder ] , 
     1   AS   [ C1 ]
     FROM   [ dbo ] . [ Products ]   AS   [ Extent1 ]
)   AS   [ Project1 ]
ORDER   BY   [ Project1 ] . [ UnitPrice ]   ASC

OrderByDescending

using  (var ctx  =   new  NorthwindEntities())
{
    // OrderByDescending 对应的 Linq 方法
    var p1 = ctx.Products.OrderByDescending(p => p.UnitPrice);
    p1.ToList();

    // OrderByDescending 对应的查询语法
    var p2 = from p in ctx.Products orderby p.UnitPrice descending select p;
    p2.ToList();
}

-- OrderByDescending 对应的 sql 语句

SELECT  
[ Project1 ] . [ C1 ]   AS   [ C1 ] , 
[ Project1 ] . [ Discontinued ]   AS   [ Discontinued ] , 
[ Project1 ] . [ ProductID ]   AS   [ ProductID ] , 
[ Project1 ] . [ ProductName ]   AS   [ ProductName ] , 
[ Project1 ] . [ QuantityPerUnit ]   AS   [ QuantityPerUnit ] , 
[ Project1 ] . [ ReorderLevel ]   AS   [ ReorderLevel ] , 
[ Project1 ] . [ UnitPrice ]   AS   [ UnitPrice ] , 
[ Project1 ] . [ UnitsInStock ]   AS   [ UnitsInStock ] , 
[ Project1 ] . [ UnitsOnOrder ]   AS   [ UnitsOnOrder ] , 
[ Project1 ] . [ CategoryID ]   AS   [ CategoryID ] , 
[ Project1 ] . [ SupplierID ]   AS   [ SupplierID ]
FROM  (  SELECT  
     [ Extent1 ] . [ CategoryID ]   AS   [ CategoryID ] , 
     [ Extent1 ] . [ Discontinued ]   AS   [ Discontinued ] , 
     [ Extent1 ] . [ ProductID ]   AS   [ ProductID ] , 
     [ Extent1 ] . [ ProductName ]   AS   [ ProductName ] , 
     [ Extent1 ] . [ QuantityPerUnit ]   AS   [ QuantityPerUnit ] , 
     [ Extent1 ] . [ ReorderLevel ]   AS   [ ReorderLevel ] , 
     [ Extent1 ] . [ SupplierID ]   AS   [ SupplierID ] , 
     [ Extent1 ] . [ UnitPrice ]   AS   [ UnitPrice ] , 
     [ Extent1 ] . [ UnitsInStock ]   AS   [ UnitsInStock ] , 
     [ Extent1 ] . [ UnitsOnOrder ]   AS   [ UnitsOnOrder ] , 
     1   AS   [ C1 ]
     FROM   [ dbo ] . [ Products ]   AS   [ Extent1 ]
)   AS   [ Project1 ]
ORDER   BY   [ Project1 ] . [ UnitPrice ]   DESC

ThenBy

using  (var ctx  =   new  NorthwindEntities())
{
    // ThenBy 对应的 Linq 方法
    var p1 = ctx.Products.OrderBy(p => p.UnitPrice).ThenBy(p => p.ProductID);
    p1.ToList();

    // ThenBy 对应的查询语法
    var p2 = from p in ctx.Products orderby p.UnitPrice ascending, p.ProductID ascending select p;
    p2.ToList();
}

-- ThenBy 对应的 sql 语句

SELECT  
[ Project1 ] . [ C1 ]   AS   [ C1 ] , 
[ Project1 ] . [ Discontinued ]   AS   [ Discontinued ] , 
[ Project1 ] . [ ProductID ]   AS   [ ProductID ] , 
[ Project1 ] . [ ProductName ]   AS   [ ProductName ] , 
[ Project1 ] . [ QuantityPerUnit ]   AS   [ QuantityPerUnit ] , 
[ Project1 ] . [ ReorderLevel ]   AS   [ ReorderLevel ] , 
[ Project1 ] . [ UnitPrice ]   AS   [ UnitPrice ] , 
[ Project1 ] . [ UnitsInStock ]   AS   [ UnitsInStock ] , 
[ Project1 ] . [ UnitsOnOrder ]   AS   [ UnitsOnOrder ] , 
[ Project1 ] . [ CategoryID ]   AS   [ CategoryID ] , 
[ Project1 ] . [ SupplierID ]   AS   [ SupplierID ]
FROM  (  SELECT  
     [ Extent1 ] . [ CategoryID ]   AS   [ CategoryID ] , 
     [ Extent1 ] . [ Discontinued ]   AS   [ Discontinued ] , 
     [ Extent1 ] . [ ProductID ]   AS   [ ProductID ] , 
     [ Extent1 ] . [ ProductName ]   AS   [ ProductName ] , 
     [ Extent1 ] . [ QuantityPerUnit ]   AS   [ QuantityPerUnit ] , 
     [ Extent1 ] . [ ReorderLevel ]   AS   [ ReorderLevel ] , 
     [ Extent1 ] . [ SupplierID ]   AS   [ SupplierID ] , 
     [ Extent1 ] . [ UnitPrice ]   AS   [ UnitPrice ] , 
     [ Extent1 ] . [ UnitsInStock ]   AS   [ UnitsInStock ] , 
     [ Extent1 ] . [ UnitsOnOrder ]   AS   [ UnitsOnOrder ] , 
     1   AS   [ C1 ]
     FROM   [ dbo ] . [ Products ]   AS   [ Extent1 ]
)   AS   [ Project1 ]
ORDER   BY   [ Project1 ] . [ UnitPrice ]   ASC ,  [ Project1 ] . [ ProductID ]   ASC

ThenByDescending

using  (var ctx  =   new  NorthwindEntities())
{
    // ThenByDescending 对应的 Linq 方法
    var p1 = ctx.Products.OrderBy(p => p.UnitPrice).ThenByDescending(p => p.ProductID);
    p1.ToList();

    // ThenByDescending 对应的查询语法
    var p2 = from p in ctx.Products orderby p.UnitPrice ascending, p.ProductID descending select p;
    p2.ToList();
}

-- ThenByDescending 对应的 sql 语句

SELECT  
[ Project1 ] . [ C1 ]   AS   [ C1 ] , 
[ Project1 ] . [ Discontinued ]   AS   [ Discontinued ] , 
[ Project1 ] . [ ProductID ]   AS   [ ProductID ] , 
[ Project1 ] . [ ProductName ]   AS   [ ProductName ] , 
[ Project1 ] . [ QuantityPerUnit ]   AS   [ QuantityPerUnit ] , 
[ Project1 ] . [ ReorderLevel ]   AS   [ ReorderLevel ] , 
[ Project1 ] . [ UnitPrice ]   AS   [ UnitPrice ] , 
[ Project1 ] . [ UnitsInStock ]   AS   [ UnitsInStock ] , 
[ Project1 ] . [ UnitsOnOrder ]   AS   [ UnitsOnOrder ] , 
[ Project1 ] . [ CategoryID ]   AS   [ CategoryID ] , 
[ Project1 ] . [ SupplierID ]   AS   [ SupplierID ]
FROM  (  SELECT  
     [ Extent1 ] . [ CategoryID ]   AS   [ CategoryID ] , 
     [ Extent1 ] . [ Discontinued ]   AS   [ Discontinued ] , 
     [ Extent1 ] . [ ProductID ]   AS   [ ProductID ] , 
     [ Extent1 ] . [ ProductName ]   AS   [ ProductName ] , 
     [ Extent1 ] . [ QuantityPerUnit ]   AS   [ QuantityPerUnit ] , 
     [ Extent1 ] . [ ReorderLevel ]   AS   [ ReorderLevel ] , 
     [ Extent1 ] . [ SupplierID ]   AS   [ SupplierID ] , 
     [ Extent1 ] . [ UnitPrice ]   AS   [ UnitPrice ] , 
     [ Extent1 ] . [ UnitsInStock ]   AS   [ UnitsInStock ] , 
     [ Extent1 ] . [ UnitsOnOrder ]   AS   [ UnitsOnOrder ] , 
     1   AS   [ C1 ]
     FROM   [ dbo ] . [ Products ]   AS   [ Extent1 ]
)   AS   [ Project1 ]
ORDER   BY   [ Project1 ] . [ UnitPrice ]   ASC ,  [ Project1 ] . [ ProductID ]   DESC

GroupBy

using  (var ctx  =   new  NorthwindEntities())
{
    // GroupBy 对应的 Linq 方法
    var p1 = ctx.Products.GroupBy(p => p.Suppliers.SupplierID).Select(g => new { Group = g.Key, Member = g });
    foreach (var g in p1)
    {
        // g.Group - 供货商ID，以此分组
        // g.Member - 某供货商下的所有产品
        foreach (var m in g.Member)
        {
            // code
        }
    }

    // GroupBy 对应的查询语法
    var p2 = from p in ctx.Products group p by p.Suppliers.SupplierID into g select new { Group = g.Key, Member = g };
    foreach (var g in p2)
    {
        // g.Group - 供货商ID，以此分组
        // g.Member - 某供货商下的所有产品
        foreach (var m in g.Member)
        {
            // code
        }
    }
}

-- GroupBy 对应的 sql 语句

SELECT  
[ Project2 ] . [ SupplierID ]   AS   [ SupplierID ] , 
[ Project2 ] . [ C1 ]   AS   [ C1 ] , 
[ Project2 ] . [ C2 ]   AS   [ C2 ] , 
[ Project2 ] . [ C4 ]   AS   [ C3 ] , 
[ Project2 ] . [ C3 ]   AS   [ C4 ] , 
[ Project2 ] . [ Discontinued ]   AS   [ Discontinued ] , 
[ Project2 ] . [ ProductID ]   AS   [ ProductID ] , 
[ Project2 ] . [ ProductName ]   AS   [ ProductName ] , 
[ Project2 ] . [ QuantityPerUnit ]   AS   [ QuantityPerUnit ] , 
[ Project2 ] . [ ReorderLevel ]   AS   [ ReorderLevel ] , 
[ Project2 ] . [ UnitPrice ]   AS   [ UnitPrice ] , 
[ Project2 ] . [ UnitsInStock ]   AS   [ UnitsInStock ] , 
[ Project2 ] . [ UnitsOnOrder ]   AS   [ UnitsOnOrder ] , 
[ Project2 ] . [ CategoryID ]   AS   [ CategoryID ] , 
[ Project2 ] . [ SupplierID1 ]   AS   [ SupplierID1 ]
FROM  (  SELECT  
     [ Distinct1 ] . [ SupplierID ]   AS   [ SupplierID ] , 
     1   AS   [ C1 ] , 
     1   AS   [ C2 ] , 
     CASE   WHEN  ( [ Extent2 ] . [ Discontinued ]   IS   NULL )  THEN   CAST ( NULL   AS   int )  ELSE   1   END   AS   [ C3 ] , 
     [ Extent2 ] . [ CategoryID ]   AS   [ CategoryID ] , 
     [ Extent2 ] . [ Discontinued ]   AS   [ Discontinued ] , 
     [ Extent2 ] . [ ProductID ]   AS   [ ProductID ] , 
     [ Extent2 ] . [ ProductName ]   AS   [ ProductName ] , 
     [ Extent2 ] . [ QuantityPerUnit ]   AS   [ QuantityPerUnit ] , 
     [ Extent2 ] . [ ReorderLevel ]   AS   [ ReorderLevel ] , 
     [ Extent2 ] . [ SupplierID ]   AS   [ SupplierID1 ] , 
     [ Extent2 ] . [ UnitPrice ]   AS   [ UnitPrice ] , 
     [ Extent2 ] . [ UnitsInStock ]   AS   [ UnitsInStock ] , 
     [ Extent2 ] . [ UnitsOnOrder ]   AS   [ UnitsOnOrder ] , 
     CASE   WHEN  ( [ Extent2 ] . [ Discontinued ]   IS   NULL )  THEN   CAST ( NULL   AS   int )  ELSE   1   END   AS   [ C4 ]
     FROM    ( SELECT   DISTINCT  
         [ Extent1 ] . [ SupplierID ]   AS   [ SupplierID ]
         FROM   [ dbo ] . [ Products ]   AS   [ Extent1 ]  )  AS   [ Distinct1 ]
     LEFT   OUTER   JOIN   [ dbo ] . [ Products ]   AS   [ Extent2 ]   ON  ( [ Extent2 ] . [ SupplierID ]   =   [ Distinct1 ] . [ SupplierID ] )  OR  (( [ Extent2 ] . [ SupplierID ]   IS   NULL )  AND  ( [ Distinct1 ] . [ SupplierID ]   IS   NULL ))
)   AS   [ Project2 ]
ORDER   BY   [ Project2 ] . [ SupplierID ]   ASC ,  [ Project2 ] . [ C4 ]   ASC

Join

using  (var ctx  =   new  NorthwindEntities())
{
    // Join 对应的 Linq 方法
    var p1 = ctx.Products.Join(
        ctx.Categories,
        p => p.Categories.CategoryID,
        c => c.CategoryID,
        (p, c) => new { c.CategoryName, p.ProductName });
    p1.ToList();

    // Join 对应的查询语法
    var p2 = from p in ctx.Products
             join c in ctx.Categories
             on p.Categories.CategoryID equals c.CategoryID
             select new { c.CategoryName, p.ProductName };
    p2.ToList();
}

-- Join 对应的 sql 语句

SELECT  
1   AS   [ C1 ] , 
[ Extent2 ] . [ CategoryName ]   AS   [ CategoryName ] , 
[ Extent1 ] . [ ProductName ]   AS   [ ProductName ]
FROM    [ dbo ] . [ Products ]   AS   [ Extent1 ]
INNER   JOIN   [ dbo ] . [ Categories ]   AS   [ Extent2 ]   ON  ( [ Extent1 ] . [ CategoryID ]   =   [ Extent2 ] . [ CategoryID ] )  OR  (( [ Extent1 ] . [ CategoryID ]   IS   NULL )  AND  ( [ Extent2 ] . [ CategoryID ]   IS   NULL ))

GroupJoin

using  (var ctx  =   new  NorthwindEntities())
{
    // GroupJoin 对应的 Linq 方法
    var p1 = ctx.Categories.GroupJoin(
        ctx.Products,
        c => c.CategoryID,
        p => p.Categories.CategoryID,
        (p, g) => new { p.CategoryName, ProductCount = g.Count() });
    p1.ToList();

    // GroupJoin 对应的查询语法
    var p2 = from c in ctx.Categories
             join p in ctx.Products
             on c.CategoryID equals p.Categories.CategoryID into g // g - IEnumerable<Products>
             select new { CategoryName = c.CategoryName, ProductCount = g.Count() };
    p2.ToList();
}

-- GroupJoin 对应的 sql 语句

SELECT  
1   AS   [ C1 ] , 
[ Project1 ] . [ CategoryName ]   AS   [ CategoryName ] , 
[ Project1 ] . [ C1 ]   AS   [ C2 ]
FROM  (  SELECT  
     [ Extent1 ] . [ CategoryName ]   AS   [ CategoryName ] , 
    ( SELECT  
         COUNT ( cast ( 1   as   bit ))  AS   [ A1 ]
         FROM   [ dbo ] . [ Products ]   AS   [ Extent2 ]
         WHERE  ( [ Extent1 ] . [ CategoryID ]   =   [ Extent2 ] . [ CategoryID ] )  OR  (( [ Extent1 ] . [ CategoryID ]   IS   NULL )  AND  ( [ Extent2 ] . [ CategoryID ]   IS   NULL )))  AS   [ C1 ]
     FROM   [ dbo ] . [ Categories ]   AS   [ Extent1 ]
)   AS   [ Project1 ]

OK
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