如何连接多个表？

Posted 2023-04-18

【中文标题】如何连接多个表？

【英文标题】：How to join multiple tables?

【发布时间】：2012-06-27 14:18:00

【问题描述】：

我有以下课程。我有一个描述类的对象var。我想使用 Linq to Sql 或 Lambda 表达式选择与 var 对象中提供的客户端相关的余额。如何连接这些表以从 Account 表中获取余额？

public class Description
    
        public int DescriptionID  get; set; 

       // Attributes

        public int ClientID  get; set; 

        [ForeignKey("ClientID")]
        public virtual Client Client  get; set; 


    

public class Client
    
        public int ClientID  get; set; 

       // Attributes

        public int UserID  get; set; 

        [ForeignKey("UserID")]
        public virtual User User  get; set; 

    

 public class User
    
        public int UserID  get; set; 

       // Attributes

     

 public class Account
    

        public int AccountID  get; set; 

        [Required, Column("Balance"), Display(Name = "Account Balance")]
        public double Balance  get; set; 


        public int UserID  get; set; 

        [ForeignKey("UserID")]
        public virtual User User  get; set; 

    

【问题讨论】：

***.com/questions/21051612/…

这能回答你的问题吗？ Entity Framework Join 3 Tables

【参考方案1】：

你可以试试这个：

var balance = (from a in context.Accounts
               join c in context.Clients on a.UserID equals c.UserID
               where c.ClientID == yourDescriptionObject.ClientID
               select a.Balance)
              .SingleOrDefault();

或者 - 如果你只有 DescriptionID:

var balance = (from a in context.Accounts
               join c in context.Clients on a.UserID equals c.UserID
               join d in context.Descriptions on c.ClientID equals d.ClientID
               where d.DescriptionID == yourDescriptionID
               select a.Balance)
              .SingleOrDefault();

（或FirstOrDefault() 或ToList() 或Sum()？因为您的模型将允许客户/描述与多个帐户相关...）