求助关于C#中的SQL问题！！！

"select * from dbo.Student where Student_id in(select Student_id from dbo.Student_Course where Course_id=(select Course_id from dbo.Course where Course_name ='"+ this.comboBox2.Text +"')) ";  这句话有问题吗？


private void Form2_Load(object sender, EventArgs e)
        {
         
            string constr = @"Integrated Security=SSPI;Persist Security Info=False;Initial Catalog=学生系统;Data Source=(local)";
            SqlConnection conn = new SqlConnection(constr);
            DataSet ds = new DataSet();
            try
            {
                string sqlstr = "select * from dbo.Users  "
                    + "select * from dbo.Student  "
                    + "select * from dbo.Course  "
                    + "select * from dbo.Student where Student_id in(select Student_id from dbo.Student_Course where Course_id=(select Course_id from dbo.Course where Course_name ='"+ this.comboBox2.Text