Question

我已经定义了两个映射到数据库中两个表的实体。在SQL中我会像这样进行连接：

select *
from tableA a
left outer join tableB b on b.ID = a.ID
where some condition

如何使用LINQ查询执行此操作？

Answer 1

使用Lambda表达式，您可以使用GroupJoin

示例：

var query =
  People
  .GroupJoin(
    Pets,
    person => person.PersonId,
    pet => per.Owner,
    (person, petCollection) =>
       new
       {
          Person = person,
          Pets = petCollection.Select(pet => pet.Name),
       });

Answer 2

请参阅MSDN上的How to: Perform Left Outer Joins (C# Programming Guide)。

例如：

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 == null ? String.Empty : subpet.Name) };

如何在LINQ中进行外连接？

2 个答案: