在 LINQ 语句中执行多个连接

Question

谁能帮我把下面的 SQL 查询翻译成 LINQ 格式。

SELECT a.ID,
       a.HostID,
       h.URL,
       a.SourceURL,
       a.TargetURL,
       c.Value,
       a.ExtFlag
FROM Link a
INNER JOIN Host h
ON h.ID = a.HostID
INNER JOIN Ref c
ON a.ResponseCode = c.SubType
AND c.Type = 'HTTP Status'

非常感谢

Answer 1

我想应该是这样的：

var result = from a in Context.DGApprovedLink 
             join h in Context.DGHost on a.HostID equals h.ID
             join c in Context.DGConfig on a.ResponseCode equals c.SubType
             where c.Type == "HTTP Status"
             select new {
                 a.ID,
                 a.HostID,
                 h.URL,
                 a.SourceURL,
                 a.TargetURL,
                 c.Value,
                 a.ExtFlag };

Answer 2

Create Unit test class using MStest and copy the code 

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace LinqTest.Test
{

    public class Employee
    {
        public int EmpId { get; set; }
        public string Name { get; set; }
        public DateTime? DOB { get; set; }
        public decimal Salary { get; set; }
        public DateTime DOJ { get; set; }
        public bool IsActive { get; set; }
    }


    public class Book
    {
        public int BookId { get; set; }
        public string Title { get; set; }
        public double Price { get; set; }
    }

    public class BookOrder
    {
        public int OrderId { get; set; }
        public int EmpId { get; set; }
        public int BookId { get; set; }
        public int Quantity { get; set; }
    }



    [TestClass]
    public class Linqtest
    {
        List<Employee> Employees;
        List<Book> Books;
        List<BookOrder> Orders;

        [TestInitialize]
        public void InitializeData()
        {
            Employees = new List<Employee>();
            Books = new List<Book>();
            Orders = new List<BookOrder>();

            Employees.Add(new Employee(){EmpId = 1, Name ="Test1" ,  DOB = new DateTime(1980,12,15),IsActive = true,Salary = 4500});
            Employees.Add(new Employee() { EmpId = 11, Name = "Test2", DOB = new DateTime(1981, 12, 15), IsActive = true, Salary = 3500 });
            Employees.Add(new Employee() { EmpId = 5, Name = "Test3", DOB = new DateTime(1970, 2, 15), IsActive = true, Salary = 5500 });
            Employees.Add(new Employee() { EmpId = 8, Name = "Test4", DOB = new DateTime(1978, 1, 15), IsActive = true, Salary = 7500 });
            Employees.Add(new Employee() { EmpId = 9, Name = "Test5", DOB = new DateTime(1972, 2, 5), IsActive = true, Salary = 2500 });
            Employees.Add(new Employee() { EmpId = 10, Name = "Test6", DOB = new DateTime(1980, 10, 8), IsActive = false, Salary = 5500 });
            Employees.Add(new Employee() { EmpId = 15, Name = "Test7", DOB = new DateTime(1983, 11, 25), IsActive = true, Salary = 3500 });

            Books.Add(new Book(){BookId = 2, Price = 24.99,Title = "British Food"});
            Books.Add(new Book() { BookId = 5, Price = 4.99, Title = "Holidays in UK" });
            Books.Add(new Book() { BookId = 7, Price = 7.99, Title = "UK Laws" });


            Orders.Add(new BookOrder(){EmpId = 1,OrderId = 1,BookId = 2,Quantity = 3});
            Orders.Add(new BookOrder() { EmpId = 1, OrderId = 1, BookId = 5, Quantity = 1 });
            Orders.Add(new BookOrder() { EmpId = 1, OrderId = 2, BookId = 7, Quantity = 5 });
            Orders.Add(new BookOrder() { EmpId = 11, OrderId = 3, BookId = 2, Quantity = 3 });
            Orders.Add(new BookOrder() { EmpId = 11, OrderId = 4, BookId = 7, Quantity = 3 });
        }


        [TestMethod]
        public void CheckEmpCount()
        {

            var res = Employees
                .Where(e => e.EmpId > 5)
                .Where(t =>t.Salary>=5000);

            Assert.AreEqual(2,res.Count());

            res = Employees
                .Where(e => e.EmpId > 5);

            Assert.AreEqual(5,res.Count());

        }

        [TestMethod]
        public void TestGroupBy()
        {
            var res = from e in Employees
                group e by e.Salary;

            Assert.AreEqual(5,res.Count());

            var res1 = Employees.GroupBy(e => e.Salary);
            Assert.AreEqual(5, res1.Count());
        }

        [TestMethod]
        public void TestJoin()
        {
            var res = from o in Orders
                join Employee e in Employees
                    on o.EmpId equals e.EmpId
                where o.EmpId == 11
                select o;

            Assert.AreEqual(2,res.Count());
        }

        [TestMethod]
        public void TestJoinData()
        {
            var res = from o in Orders
                join Employee e in Employees
                    on o.EmpId equals e.EmpId 
                join Book b in Books
                    on o.BookId equals b.BookId 
                orderby e.EmpId
                select new {o.OrderId, e.Name, b.Title, b.Price};

            Assert.AreEqual("Test1", res.First().Name);

        }

    }
}