Linq.join_lambda知识点

Posted 2020-11-10 csskill

1、一直  不太明白 join的 lambda的写法，先记录一些我查到的东西：

　1.1、有关 Linq 与 Lambda 的Join连接查询_已解决_博问_博客园.html（https://q.cnblogs.com/q/106310/）

　　ZC：里面有人说 “lambda 太长 比linq 恶心多了  短的时候用lambda  长的时候用linq”

　1.2、c# - Join_Where with LINQ and Lambda - Stack Overflow.html（https://stackoverflow.com/questions/2767709/join-where-with-linq-and-lambda）

　　ZC：里面的 被接受的回复：（ZC：可以对比一下 回复者的2中写法）

　　　I find that if you\'re familiar with SQL syntax, using the LINQ query syntax is much clearer, more natural, and makes it easier to spot errors:

　　　var id = 1;
　　　var query =
   　　　from post in database.Posts
   　　　join meta in database.Post_Metas on post.ID equals meta.Post_ID
   　　　where post.ID == id
   　　　select new { Post = post, Meta = meta };

　　　If you\'re really stuck on using lambdas though, your syntax is quite a bit off. Here\'s the same query, using the LINQ extension methods:

　　　var id = 1;
　　　var query = database.Posts    // your starting point - table in the "from" statement
   　　　.Join(database.Post_Metas, // the source table of the inner join
     　　　　post => post.ID,        // Select the primary key (the first part of the "on" clause in an sql "join" statement)
     　　　　meta => meta.Post_ID,   // Select the foreign key (the second part of the "on" clause)
     　　　　(post, meta) => new { Post = post, Meta = meta }) // selection
   　　　.Where(postAndMeta => postAndMeta.Post.ID == id);    // where statement

 

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