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）：
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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/>");
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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：
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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]
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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：
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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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查询经过改造后会得到下面的SQL：
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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]
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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：
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BoardCategory dbcat = new BoardCategory() { CategoryName = "Database" };
Board oracle = new Board() { BoardName = "Oracle", Category = dbcat};
ctx.BoardCategories.Add(dbcat);
ctx.SubmitChanges();
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上述代码导致下面的SQL被执行：
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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]
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