linq join用法，左连接

单条件：

var query = from person in people
join pet in pets on person equals pet.Owner
select new { OwnerName = person.FirstName, PetName = pet.Name };

多条件：

IEnumerable<string> query = from employee in employees
join student in students
on new { employee.FirstName, employee.LastName }
equals new { student.FirstName, student.LastName }
select employee.FirstName + " " + employee.LastName;

左连接：

var list = from a in db.tab1
join b in db.tab2 on a.Id equals b.Id into bb
from bbdata in bb.DefaultIfEmpty()
select new
{
info1=a,
info2 = bbdata ,
};

db.tab2就是右表了，等于：select * from tab1 as a left join tab2 as b on a.Id=b.Id

db.Products
.GroupJoin(db.Categories,
          (Product p) => p.CategoryId,
          (Category c) => c.CategoryId,
          (prod, cs) => new { prod, cs }) // cs is IEnumerable<Category>
          .SelectMany(prodCats => prodCats.cs.DefaultIfEmpty(), (prodCats, c) =>
                                new
                                {
                                    prodCats.prod
                                    categoryName = c.CategoryName
                                });

https://www.cnblogs.com/ybtools/p/6559292.html
https://www.cnblogs.com/zitjubiz/p/a-left-outer-join-using-linq-extension-methods.html