如何将一个表列的行复制到另一个表列

我想从表中复制多行exam_sections并将其插入到exam_students。但是我对如何让它工作感到困惑。我在 WHERE 子句中有一个错误。有人可以帮助我吗？顺便说一句，对不起我的英语不好。

这是我的代码：

if(isset($_POST['Add'])){
    session_start();
    $year = $_POST['year'];
    $course = $_POST['course'];
    $block = $_POST['block'];
    $examinergroupid = $_SESSION['usergroupid'];
    $subject_id = $_SESSION['subject_id'];
    $add = mysql_query("
INSERT INTO 
exam_students (stud_valid_nos, stud_email, stud_subject_id, stud_group_id )
SELECT 
exam_sections.sec_id_num,
exam_sections.sec_email, 
'$subject_id','$examinergroupid'  
WHERE exam_sections.sec_year = '$year'
AND exam_sections.sec_course = '$course'
AND exam_sections.sec_block = '$block'")
or die(mysql_error());

    }