Linq to objects（二）

DataTable products = myDataSet.Tables["products"];
DataTable classes = myDataSet.Tables["classes"];

内部连接

var result =
	from s in products
	// Inner Join表products和表classes，注意equals关键字
	join c in classes on s.cid equals c.id
	select new {
		id = s.id,          // 商品id
		name = s.name,      // 商品名称
		cname = c.name      // 分类名
	};

分组连接

var result =
	from s in products
	// 实际上就是内连接后按照cid进行分组
	join c in classes on s.cid equals c.id into G
	select new {
		id = G.Key,     // 分组的cid
		name = c.name,  // 分类名
		list = G        // 组成员，组成员为连接后的记录，字段要比之前多
	};

左外连接

var result =
	from c in classes
	join s in products on c.id = s.cid into G
	from item in G.DefaultIfEmpty()		// 当右侧products表无法匹配左表classes时，赋null
	select new {
		cname = c.name,
		list = item;	// item可能为null
	};

为null的item赋默认值

var result =
	from c in classes
	join s in products on c.id = s.cid into G
	from item in G.DefaultIfEmpty(new product{id=0, name="", cid=0})		// 当右侧products表无法匹配左表classes时，赋新建的product对象
	select new {
		cname = c.name,
		list = item;
	};

转载于:https://www.cnblogs.com/rainman/archive/2013/06/01/3111757.html