使用 Linq 进行左连接查询

使用 linq 进行左连接查询

Intro

今天改项目发现一个bug，原因是因为两个数据 join 的时候另外一个数据不存在，所以导致最后join出来的结果和预期不符，于是要改成左连接查询，突然发现自己不会写 linq 的左连接查询。。

Linq 左连接

默认join 的是属于是 inner join，如果要使用到左连接，你需要使用 DefaultIfEmpty 来在没有join到数据的时候返回默认的集合

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

class Pet
{
    public string Name { get; set; }
    public Person Owner { get; set; }
}

public static void LeftOuterJoinExample()
{
    Person magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" };
    Person terry = new Person { FirstName = "Terry", LastName = "Adams" };
    Person charlotte = new Person { FirstName = "Charlotte", LastName = "Weiss" };
    Person arlene = new Person { FirstName = "Arlene", LastName = "Huff" };

    Pet barley = new Pet { Name = "Barley", Owner = terry };
    Pet boots = new Pet { Name = "Boots", Owner = terry };
    Pet whiskers = new Pet { Name = "Whiskers", Owner = charlotte };
    Pet bluemoon = new Pet { Name = "Blue Moon", Owner = terry };
    Pet daisy = new Pet { Name = "Daisy", Owner = magnus };

    // Create two lists.
    List people = new List { magnus, terry, charlotte, arlene };
    List pets = new List { barley, boots, whiskers, bluemoon, daisy };

    var query = from person in people
                join pet in pets on person equals pet.Owner into gj
                from subpet in gj.DefaultIfEmpty()
                select new { person.FirstName, PetName = subpet?.Name ?? String.Empty };

    foreach (var v in query)
    {
        Console.WriteLine($"{v.FirstName+":",-15}{v.PetName}");
    }
}

// This code produces the following output:
//
// Magnus:        Daisy
// Terry:         Barley
// Terry:         Boots
// Terry:         Blue Moon
// Charlotte:     Whiskers
// Arlene:

Reference

• https://docs.microsoft.com/en-us/dotnet/csharp/linq/perform-left-outer-joins
• https://docs.microsoft.com/zh-cn/dotnet/csharp/linq/perform-left-outer-joins