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;
};
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