MySQL: select one table from another table

Suppose MySQL tables edu_courses and edu_extra1_study have a smae primary key: course_id, to select from edu_courses based on the condition in edu_extra1_study, we can use following MySQL statement if requiring order by title:
SELECT * FROM edu_courses   WHERE course_id In (SELECT course_id FROM edu_extra1_study) ORDER BY title;
If  edu_extra1_study selection is based on the variable in url
data_export.php?c1=1&c2=0&c3=0
where c1, c2 and c3 from checkbox, i.e, the value equal to if  the checkbox is checked.
We we defined an arry in PHP:
$q1= Array();
If the checkbox is checked., we can add a condition, such as:
$q1[]='competency=3';
 finally we can use PHP implode to join all the conditions:
implode(' AND ',$q1);
Final code:
<?php
$querystring='SELECT course_id FROM edu_extra1_study  ';
$q1= Array();
if(isset($_GET['c1'])){
 if($_GET['c1']==1) { $q1[]='competency=3';}
 }
 if(isset($_GET['c2'])){
 if($_GET['c2']==1) {$q1[]='competency=2';}
 }
if(isset($_GET['c3'])){
 if($_GET['c3']==1) {$q1[]='cognitive=1';}
 }
if(!empty($q1))$querystring=$querystring.'WHERE '.implode(' AND ',$q1);
if(empty($q1))
{$sql = "SELECT * FROM edu_courses ORDER BY title";}
else
{$sql = "SELECT * FROM edu_courses WHERE course_id in (".$querystring.")ORDER BY title";}
?>