如何使用实体框架和 linq 仅获取具有唯一属性的记录？

Question

public class Person
{
     public string firstName;
     public string lastName;
}

我想要一个具有唯一名字的所有人员的列表。

人员表

Tom Haverford
Tom Baker
Amy Pond
Amy Santiago
Trish Walker
Chidi Anagonye

查询应该返回

Trish, Chidi

我尝试过使用 Distinct 以及 GroupBy 和 Select 的组合，但它们返回 Trish, Chidi, Tom, Amy。

Answer 1

Demo on dotnet fiddle

您可以Group by 然后count 重复项目的数量。之后，你可以得到count值等于1的项目，如下所示。

    var arr = new []
    {
        new Person {    firstName = "Tom", lastName = "Haverford" },
        new Person {    firstName = "Tom", lastName = "Baker"},
        new Person {    firstName = "Amy", lastName = "Pond" },
        new Person {    firstName = "Amy", lastName = "Santiago"},
        new Person {    firstName = "Trish", lastName = "Walker"},
        new Person {    firstName = "Chidi", lastName ="Anagonye" }
    };

    var result = arr.GroupBy(p => p.firstName).Select(g => new { Name = g.Key, Count = g.Count()});
    foreach(var item in result.Where(p => p.Count == 1))
      Console.WriteLine(item.Name); 

输出

Trish
Chidi

Answer 2

为此，您可以同时使用 group by 和 count 功能：

1.从 DB 中获取所有人的列表：

var personList = (from p in db.Person select p).ToList(); // I assumed here that your db obj name is 'db' and table name is 'Person'

2。现在按查询应用分组以获取名字的计数：

var q = from x in personList
    group x by x.firstName into g
    let count = g.Count()
    select new {Count = count, Name = g.First().firstName }; 

3。现在你可以得到这样的最终结果：

var finalResult = (from p in q where p.Count == 1 select p).ToList();

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