Linq to sql：继承与关系-技术开发专区

Linq to sql：继承与关系

作者：lovecherry博客编辑： nancy 2008-01-25 00:00

在这里我们需要关联分类，设置了Category属性使用BoardCategory字段和分类表关联。

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-------------<br/>");
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 + "<br/>");
Response.Write("-------------查询版块大于2个的分类-------------<br/>");
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 + "<br/>");

在数据库中加一些测试数据，如下图：

运行程序后得到下图的结果：

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-------------<br/>");
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 + "<br/>");

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