前提：一个学生表Student，一个描述表Desc
表数据：
left join：
sql：
SELECT * FROM Student
LEFT JOIN [Desc]
ON Student.ID=[Desc].ID
linq：
读取全部：
from student in db.Student join desc in db.Desc on student.ID equals desc.StudentID into jointemp from temp in jointemp.DefaultIfEmpty() select new { student, temp };
读取指定字段：
from student in db.Student join desc in db.Desc on student.ID equals desc.StudentID into jointemp from temp in jointemp.DefaultIfEmpty()select new{ID =student.ID,Name =student.Name,Mark =temp.Mark};
lambda：
读取全部：
db.Student.GroupJoin(db.Desc, u => u.ID, d => d.StudentID, (u, d) => new { u, d }).Select(o => o)；
读取指定字段：
db.Student.GroupJoin(db.Desc, u => u.ID, d => d.StudentID, (u, d) => d.DefaultIfEmpty().Select(p => new {ID=u.ID,Name=u.Name,Mark=p.Mark })).SelectMany(o => o);
right join：
SELECT * FROM Student
RIGHTJOIN [Desc]
ON Student.ID=[Desc].ID
PS：A left join B=B right join A
对应linq以及lambda即调换两个表位置
inner join：
SELECT * FROM Student
INNER JOIN [Desc]
ON Student.ID=[Desc].ID
linq:
读取全部：
from student in db.Student join desc in db.Desc on student.ID equals desc.StudentID select new { student, desc };
读取指定字段：
from student in db.Student join desc in db.Desc on student.ID equals desc.StudentID select new { ID = student.ID, Name = student.Name, Mark = desc.Mark };
lambda:
读取全部：
db.Student.Join(db.Desc, u => u.ID, d => d.StudentID, (u, d) => new { u, d }).Select(o => o);
读取指定字段：
db.Student.Join(db.Desc, u => u.ID, d => d.StudentID, (u, d) =>new { ID = u.ID, Name = u.Name, Mark = d.Mark }).Select(o => o);
full join：
SELECT * FROM Student
FULL JOIN [Desc]
ON Student.ID=[Desc].ID
corss join：
SELECT * FROM Student
CORSS JOIN [Desc]
linq：
读取全部
from student in db.Student from desc in db.Desc select new { student, desc };
读取指定字段
from student in db.Student from desc in db.Desc select new { ID = student.ID, Name = student.Name, Mark = desc.Mark };
lambda：
读取全部
读取指定字段
