关于数据库的查询中经常需要用到多表的连接查询，这里就简单地展示关于linq的查询功能。1、单表的查询[csharp] view plain copy    var query = from tc in db.tbClass               where tc.ClassID == "1"                  //查询表tbClass              select new {              ClassID=tc.ClassID,              ClassName=tc.ClassName                  }  2、多表内连接查询[csharp] view plain copy    var query = from s in db.tbStudents                                  join c in db.tbClass on s.ClassID equals c.ClassID                                  where s.ClassID == 3                                  select new                                  {                                      ClassID = s.ClassID,                                      ClassName = c.ClassName,                                      Student = new                                      {                                          Name = s.Name,                                          StudentID = s.StudentID                                      }                                  };  内连接与SqL中inner join一样，即找出两个序列的交集。3、外连接[csharp] view plain copy    var query = from s in db.tbStudents                                  join c in db.tbClass on s.ClassID equals c.ClassID into tbC                                  from tbCw in tbC.DefaultIfEmpty()                                  where s.ClassID == 3                                  select new                                  {                                      ClassID = s.ClassID,                                      ClassName = tbCw.ClassName,                                      Student = new                                      {                                          Name = s.Name,                                          StudentID = s.StudentID                                      }                                  };  注意点在与外连接的时候 from外接表的时候需要进行into到新的变量中，然后进行调用DefaultIfEmpty（）方法。