Linq to sql（八）：继承与关系（五）

实体关系的使用
好了，现在我们就可以在查询句法中直接关联表了（数据库中不一定要设置表的外键关系）：

Response.Write("-------------查询分类为1的版块------------- ");

var query1 = from b in ctx.Boards where b.Category.CategoryID == 1 select b;

foreach (Board b in query1)

Response.Write(b.BoardID + " " + b.BoardName + " ");

Response.Write("-------------查询版块大于2个的分类------------- ");

var query2 = from c in ctx.BoardCategories where c.Boards.Count > 2 select c;

foreach (BoardCategory c in query2)

Response.Write(c.CategoryID + " " + c.CategoryName + " " + c.Boards.Count + " ");

在数据库中加一些测试数据，如下图：

Linq to sql（八）：继承与关系（五）

运行程序后得到下图的结果：

Linq to sql（八）：继承与关系（五）

我想定义实体关系的方便我不需要再用语言形容了吧。执行上述的程序会导致下面SQL的执行：

SELECT [t0].[BoardID], [t0].[BoardName], [t0].[BoardCategory]

FROM [Boards] AS [t0]

INNER JOIN [Categories] AS [t1] ON [t1].[CategoryID] = [t0].[BoardCategory]

WHERE [t1].[CategoryID] = @p0

-- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [1]

SELECT [t0].[CategoryID], [t0].[CategoryName]

FROM [Categories] AS [t0]

WHERE ((

SELECT COUNT(*)

FROM [Boards] AS [t1]

WHERE [t1].[BoardCategory] = [t0].[CategoryID]

)) > @p0

-- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [2]

SELECT [t0].[BoardID], [t0].[BoardName], [t0].[BoardCategory]

FROM [Boards] AS [t0]

WHERE [t0].[BoardCategory] = @p0

-- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [1]

可以看到，第二个查询并没有做外连接，还记得DataLoadOptions吗？我们可以要求Linq to sql在

读取版块分类信息的时候也把版块信息一起加载：

DataLoadOptions options = new DataLoadOptions();

options.LoadWith<BoardCategory>(c => c.Boards);

ctx.LoadOptions = options;

Response.Write("-------------查询版块大于2个的分类------------- ");

var query2 = from c in ctx.BoardCategories where c.Boards.Count > 2 select c;

foreach (BoardCategory c in query2)

Response.Write(c.CategoryID + " " + c.CategoryName + " " + c.Boards.Count + " ");

查询经过改造后会得到下面的SQL：

SELECT [t0].[CategoryID], [t0].[CategoryName], [t1].[BoardID], [t1].[BoardName], [t1].[BoardCategory], (

SELECT COUNT(*)

FROM [Boards] AS [t3]

WHERE [t3].[BoardCategory] = [t0].[CategoryID]

) AS [count]

FROM [Categories] AS [t0]

LEFT OUTER JOIN [Boards] AS [t1] ON [t1].[BoardCategory] = [t0].[CategoryID]

WHERE ((

SELECT COUNT(*)

FROM [Boards] AS [t2]

WHERE [t2].[BoardCategory] = [t0].[CategoryID]

)) > @p0

ORDER BY [t0].[CategoryID], [t1].[BoardID]

-- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [2]

在添加分类的时候，如果这个分类下还有新的版块，那么提交新增分类的时候版块也会新增：

BoardCategory dbcat = new BoardCategory() { CategoryName = "Database" };

Board oracle = new Board() { BoardName = "Oracle", Category = dbcat};

ctx.BoardCategories.Add(dbcat);

ctx.SubmitChanges();

上述代码导致下面的SQL被执行：

INSERT INTO [Categories]([CategoryName]) VALUES (@p0)

SELECT [t0].[CategoryID]

FROM [Categories] AS [t0]

WHERE [t0].[CategoryID] = (SCOPE_IDENTITY())

-- @p0: Input AnsiString (Size = 8; Prec = 0; Scale = 0) [Database]

INSERT INTO [Boards]([BoardName], [BoardCategory]) VALUES (@p0, @p1)

SELECT [t0].[BoardID]

FROM [Boards] AS [t0]

WHERE [t0].[BoardID] = (SCOPE_IDENTITY())

-- @p0: Input AnsiString (Size = 6; Prec = 0; Scale = 0) [Oracle]

-- @p1: Input Int32 (Size = 0; Prec = 0; Scale = 0) [23]