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;
}

I want a list of all Persons with a unique first name.

Persons table

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

The query should return

Trish, Chidi

I've tried using Distinct and a combination of GroupBy and Select, but those return Trish, Chidi, Tom, Amy .

Answer 1

Demo on dotnet fiddle

You can Group by then count number of duplicated items. After that, you can get the item with count value equals to 1 like below.

    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); 

Output

Trish
Chidi

Answer 2

You can use group by and count functionality together for this :

1. Get a list of all persons from DB :

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. Now apply group by query to get the count of first names :

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. Now you can get your final result like this :

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

Happy Coding...