求Ling写法,多表
select * from A left join B on A.x=B.x and A.y=B.y

C# code

            var a = (from R in Result                     join T in SuccessionGradeChains on R.GradeCode equals T.GradeCode into os                      from TT in os.DefaultIfEmpty()                     select new{                         R.GradeCode,                         R.DisciplineCode,                         SuccessorGradeNo = (TT == null ? null : TT.SuccessorGradeNo)                     });

C# code

 var a = (from R in Result                     join T in SuccessionGradeChains on new{GradeCode = R.GradeCode,GradeName=R.GradeName} equals new{ GradeCode =T.GradeCode,GradeName=T.GradeName} into os                      from TT in os.DefaultIfEmpty()                     select new{                         R.GradeCode,                         R.DisciplineCode,                         SuccessorGradeNo = (TT == null ? null : TT.SuccessorGradeNo)                     });