Linq 左连接 内连接

//左连接
from a in employeeList
let u = from b in employeeList2
        where a.ID.Equals(b.ID)
        select a
from c in u
select c;


//内连接
from a in employeeList
let u = from b in employeeList2
        where a.ID.Equals(b.ID)
        select a
from c in u.DefaultIfEmpty()
select c;