Add Two NameSpace
Using system.data;
using system.data.sqlclient;
Then write below code in to youe (!Ispostback) Function
if (!IsPostBack)
{
SqlDataAdapter ad = new SqlDataAdapter("select CourseRegistration.CourseCode, CoursesTable.CourseName, CoursesTable.CourseUnit from (DepartmentTable inner join CoursesTable on DepartmentTable.DepartmentID=CoursesTable.DepartmentID) inner join CourseRegistration on CoursesTable.CourseCode = CourseRegistration.CourseCode where MatricNo = '" + matricno + "'",cn);
DataSet ds = new DataSet();
ad.Fill(ds);
GridView1.DataSource=ds;
GridView1.DataBind();
}