inner 附近有语法错误。。看不出来 求帮助

string sql = "select sc.teachCourseNo,workerNo,workerName,courseName,c.courseNo ";
sql += " from tblTeachCourse tc ";
sql += " inner join tblTeacher t on tc.workerNo=t.workerNo ";
sql += " inner join tblCourse c on c.courseNo=tc.courseNo ";
sql += " where teachCourseNo not in(";
sql = "select sc.teachCourseNo";
sql += " from tblSelectCourse sc ";
sql += " where studentNo='" + student.getNo()+ "')";