MVC学习笔记（四）---使用linq多表联查（SQL）

1.数据库原型（Students表中的ID和Scores表中的StudentID是对应的）

2.实现效果：查询出每个学生各个科目的成绩（用的是MVC学习笔记（三）—用EF向数据库中添加数据的架构）

C#代码

        public ActionResult Index()
        {
            try
            {
                EFContextDB db = new EFContextDB();
                List<StudentScoreView> scoreList = new List<StudentScoreView>();
                scoreList = (from s in db.Students
                             join
                      c in db.Scores on s.ID equals c.StudentID
                             select new StudentScoreView
                             {
                                 ID = s.ID,
                                 Name = s.Name,
                                 Subject = c.Subject,
                                 Score = c.StudentScore,
                                 School = s.School
                             }).ToList();
                return Json(scoreList, JsonRequestBehavior.AllowGet);
            }
            catch(Exception ex)
            {
                return Json("err", JsonRequestBehavior.AllowGet);
            }
        }

加了一个显示类studentScoreView

    public class StudentScoreView
    {
        /// <summary>
        /// 主键
        /// </summary>
        public Guid ID { get; set; }
        /// <summary>
        /// 姓名
        /// </summary>
        public string Name { get; set; }
        /// <summary>
        /// 学校
        /// </summary>
        public string School { get; set; }

        /// <summary>
        /// 分数
        /// </summary>
        public int Score { get; set; }

        /// <summary>
        /// 科目
        /// </summary>
        public string Subject { get; set; }
    }

3.最后返回json结果

[{
    "ID": "5eaaff20-f806-4116-a387-39b44c4c76e0",
    "Name": "李四",
    "School": "厦门大学",
    "Score": 98,
    "Subject": "语文"
}, {
    "ID": "5eaaff20-f806-4116-a387-39b44c4c76e0",
    "Name": "李四",
    "School": "厦门大学",
    "Score": 93,
    "Subject": "数学"
}, {
    "ID": "5eaaff20-f806-4116-a387-39b44c4c76e0",
    "Name": "李四",
    "School": "厦门大学",
    "Score": 28,
    "Subject": "英语"
}, {
    "ID": "662bf2f7-3680-490c-8ff8-9208e4e5f362",
    "Name": "张三",
    "School": "北京大学",
    "Score": 88,
    "Subject": "语文"
}, {
    "ID": "662bf2f7-3680-490c-8ff8-9208e4e5f362",
    "Name": "张三",
    "School": "北京大学",
    "Score": 90,
    "Subject": "数学"
}, {
    "ID": "662bf2f7-3680-490c-8ff8-9208e4e5f362",
    "Name": "张三",
    "School": "北京大学",
    "Score": 60,
    "Subject": "英语"
}, {
    "ID": "5eaaff20-f806-4116-a387-39b44c4c76e0",
    "Name": "小明",
    "School": "中山大学",
    "Score": 91,
    "Subject": "语文"
}, {
    "ID": "4321592e-863f-411a-b044-9478952c29d1",
    "Name": "小明",
    "School": "中山大学",
    "Score": 100,
    "Subject": "数学"
}, {
    "ID": "4321592e-863f-411a-b044-9478952c29d1",
    "Name": "小明",
    "School": "中山大学",
    "Score": 92,
    "Subject": "英语"
}]

linq中多条件的写法用&&来连接

4.在实际过程中可能会遇到格式化字符串等等，按照下面方式(加上AsEnumerable())就可以了。

 1         public ActionResult Index()
 2         {
 3             try
 4             {
 5                 EFContextDB db = new EFContextDB();
 6                 List<StudentScoreView> scoreList = new List<StudentScoreView>();
 7                 scoreList = (from s in db.Students.AsEnumerable()
 8                              join
 9                       c in db.Scores.AsEnumerable() on s.ID equals c.StudentID
10                              select new StudentScoreView
11                              {
12                                  ID = s.ID,
13                                  Name = s.Name,
14                                  Subject = c.Subject,
15                                  Score = c.StudentScore,
16                                  School = s.School,
17                                  ApplyTime = i.CreateTime.ToString("yyyy-MM-dd HH:mm:ss")
18                              }).ToList();
19                 return Json(scoreList, JsonRequestBehavior.AllowGet);
20             }
21             catch(Exception ex)
22             {
23                 return Json("err", JsonRequestBehavior.AllowGet);
24             }
25         }

小插曲：中间在编译的过程中，遇到了“未能找到查询的实现”，解决方法是引用命名空间：using System.Linq

写在后面的话：不积跬步无以至千里，我喜欢用文字记录下自己学习的过程，虽然每一篇都是小知识点，每一篇都很简单，但我相信量变会引起质变，加油！