SELECT    SI_2.[StudGUID] ,SI_2.[ClassName],
SI_2.[StudGUID],SI_1.[StuName],SI_1.[Title],SI_1.[StuMobileNumber]
FROM [StudentInfo2] AS SI_2
INNER JOIN [StudentInfo] AS SI_1 ON SI_2.[StudGUID]=SI_1.[StuCode]
WHERE SI_2.[ClassID] IN 
(
    SELECT SC.[ClassID] FROM [SchoolClass] AS SC 
    LEFT JOIN [BookClass] AS BC ON SC.[ClassID]=BC.[ClassID]
    WHERE BC.[ClassID] IS NULL AND SchoolYear=2015 
)
AND SI_2.[isMonitor]=1

 var a = (from sc in context.SchoolClass.Where(s => s.ClassID == null 
&& s.SchoolYear.Equals(2015))
                    join bc in context.BookClass
                    on sc.ClassID equals bc.ClassID into temp
                    from tt in temp.DefaultIfEmpty()
                    select new
                    {
                        ClassID = sc.ClassID
                    }).ToString();
        
            query = (from s2 in context.StudentInfo2
                     join s1 in context.StudentInfo
                     on s2.StudGUID equals s1.StuCode
                     where a.Contains(s2.ClassID)
                     orderby s1.SysCreationTime descending
                     select new StudentInfoList
                     {
                         StudGUID = s2.StudGUID,
                         ClassName = s2.ClassName,
                         StuName = s1.StuName,
                         Title = s1.Title,
                         StuMobileNumber = s1.StuMobileNumber
                     }).Skip(pageIndex - 1).Take(pageSize).ToList();