LINQ方式实现学生成绩汇总（右连接、聚合函数）

需求：从数据表students中取得所有学生姓名，然后将其scores表对应中的分数相加，显示一个“姓名：分数”的列表

1. 取得所有姓名（包括没有成绩的）
2. 分数是该学生所有成绩的总和

先看效果图：

 

 

数据库结构简化模拟如下：

class Student { public int id { get; set; } public string name { get; set; } } class Score { public int StudentId { get; set; } public int score { get; set; } } 

插入测试数据：

Student p1 = new Student() { id = 1, name = "张三" }; Student p2 = new Student() { id = 2, name = "李四" }; Student p3 = new Student() { id = 3, name = "王五" }; Score s1 = new Score() { StudentId = 1, score = 80 }; Score s2 = new Score() { StudentId = 1, score = 90 }; Score s3 = new Score() { StudentId = 2, score = 70 }; List students = new List(new Student[] { p1, p2, p3 }); List scores = new List(new Score[] { s1, s2, s3 }); 

查询，注意：

1. LINQ中没有left join和right join之分，只有关键词join来控制连接，join之前表的作为完整的表连接，join之后的表需要使用DefaultIfEmpty来进行处理，才能正常的显示null值
2. group中的key是分组的字段，分组后就可以使用聚合函数了
3. LINQ就像生产流水线，可以分阶段进行数据排序、汇总、投影等操作，最后组装起来就能达到预定的效果

//left join,返回左边每一个元素 var studentsTable = from student in students join score in scores on student.id equals score.StudentId into joinedTable from j in joinedTable.DefaultIfEmpty()//处理右边为空的情况 select new { name = student.name, score = j != null ? j.score : 0 }; //聚合函数 var data = from student in studentsTable group student by student.name into g select new { name = g.Key, score = g.Sum(student => student.score) }; foreach (var d in data) { Console.WriteLine("name:" + d.name + ";score:" + d.score.ToString()); } Console.ReadLine(); 

看看效果：

foreach (var d in data) { Console.WriteLine("name:" + d.name + ";score:" + d.score.ToString()); } Console.ReadLine();