MDL-28015 workshop - fixed unique list of columns in ORDER BY clause

MSSQL requires that the columns in the ORDER BY clause list must be
unique. The patch populates the list of fields by combining an array of
explicit sort definition with an array of implicit (default) sort
definition.

mod/workshop/locallib.php

@@ -1342,7 +1342,12 @@ class workshop {
             list($participantids, $params) = $DB->get_in_or_equal(array_keys($participants), SQL_PARAMS_NAMED);
             $params['workshopid1'] = $this->id;
             $params['workshopid2'] = $this->id;
-            $sqlsort = $sortby . ' ' . $sorthow . ',u.lastname,u.firstname,u.id';
+            $sqlsort = array();
+            $sqlsortfields = array($sortby => $sorthow) + array('lastname' => 'ASC', 'firstname' => 'ASC', 'u.id' => 'ASC');
+            foreach ($sqlsortfields as $sqlsortfieldname => $sqlsortfieldhow) {
+                $sqlsort[] = $sqlsortfieldname . ' ' . $sqlsortfieldhow;
+            }
+            $sqlsort = implode(',', $sqlsort);
             $sql = "SELECT u.id AS userid,u.firstname,u.lastname,u.picture,u.imagealt,u.email,
                            s.title AS submissiontitle, s.grade AS submissiongrade, ag.gradinggrade
                       FROM {user} u