Question

我有以下表格：

Person(Id, FirstName, LastName)
{
    (1, "John", "Doe"),
    (2, "Peter", "Svendson")
    (3, "Ola", "Hansen")
    (4, "Mary", "Pettersen")
}

Sports(Id, Name)
{
    (1, "Tennis")
    (2, "Soccer")
    (3, "Hockey")
}

SportsPerPerson(Id, PersonId, SportsId)
{
    (1, 1, 1)
    (2, 1, 3)
    (3, 2, 2)
    (4, 2, 3)
    (5, 3, 2)
    (6, 4, 1)
    (7, 4, 2)
    (8, 4, 3)
}

查看表格，我们可以得出以下事实：
约翰打网球约翰扮演曲棍球彼得踢足球彼得打曲棍球奥拉踢足球玛丽打网球玛丽踢足球玛丽扮演曲棍球

现在我想创建一个Linq2Sql查询，它检索以下内容：
让所有玩曲棍球和足球的人

执行查询应返回：Peter和Mary
任何人都知道如何在Linq2Sql中解决这个问题？

Answer 1

关于Linq的一个好处是，你不必将这一切都写成一个单一的查询，因为它无论如何都要实际执行，直到你无论如何都要枚举结果。您可以编写单个查询，但。相反，您可以将其写为多个单独的查询，从而提高可读性并澄清您的意图。

var sportIds = Sports
    .Where(s => s.Name == "Hockey" || s.Name == "Soccer")
    .Select(s => s.Id);

var people = Person.Where(p => SportsPerPerson
    .Count(spp => (spp.PersonId == p.Id) 
    && sportIds.Contains(spp.SportId)) == 2);

首先，我们收集了我们感兴趣的运动项目。然后，我们在第一个列表中找到所有有两项运动的人。虽然它表示为多个查询，但当我们最终枚举结果时，Linq会将它全部压缩为一个操作。

编辑：这是一个完整的测试类，用于说明查询：

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

namespace L2STest
{
    public class Sport
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }

    public class Person
    {
        public int Id { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
    }

    public class SportPerPerson
    {
        public int Id { get; set; }
        public int PersonId { get; set; }
        public int SportId { get; set; }
    }

    [TestClass]
    public class SportsTest
    {
        private List<Person> persons;
        private List<Sport> sports;
        private List<SportPerPerson> sportsPerPerson;

         [TestInitialize]
         public void MyTestInitialize()
         {
             persons = new List<Person>
             {
                 new Person {Id = 1, FirstName = "John", LastName = "Doe"},
                 new Person {Id = 2, FirstName = "Peter", LastName = "Svendson"},
                 new Person {Id = 3, FirstName = "Ola", LastName = "Hansen"},
                 new Person {Id = 4, FirstName = "Marv", LastName = "Petterson"},
             };

             sports = new List<Sport>
             {
                 new Sport {Id = 1, Name = "Tennis"},
                 new Sport {Id = 2, Name = "Soccer"},
                 new Sport {Id = 3, Name = "Hockey"},
             };

             sportsPerPerson = new List<SportPerPerson>
             {
                 new SportPerPerson {Id = 1, PersonId = 1, SportId = 1}, 
                 new SportPerPerson {Id = 2, PersonId = 1, SportId = 3}, 
                 new SportPerPerson {Id = 3, PersonId = 2, SportId = 2}, 
                 new SportPerPerson {Id = 4, PersonId = 2, SportId = 3}, 
                 new SportPerPerson {Id = 5, PersonId = 3, SportId = 2}, 
                 new SportPerPerson {Id = 6, PersonId = 3, SportId = 1}, 
                 new SportPerPerson {Id = 7, PersonId = 4, SportId = 2}, 
                 new SportPerPerson {Id = 8, PersonId = 4, SportId = 3}, 
             };
         }

        [TestMethod]
        public void QueryTest()
        {
            var sportIds = sports
                .Where(s => s.Name == "Hockey" || s.Name == "Soccer")
                .Select(s => s.Id);

            var people = persons.Where(p => sportsPerPerson
                .Count(spp => (spp.PersonId == p.Id)
                && sportIds.Contains(spp.SportId)) == 2); 

            Assert.AreEqual(2, people.Count());
            Assert.AreEqual("Peter", people.First().FirstName);
            Assert.AreEqual("Marv", people.Last().FirstName);
        }
    }
}

如何在以下数据集上执行Linq2Sql查询

1 个答案: