diff --git a/models/TicketModel.php b/models/TicketModel.php
index c76f98f..a9cf330 100644
--- a/models/TicketModel.php
+++ b/models/TicketModel.php
@@ -151,26 +151,38 @@ class TicketModel {
         }
         
         // Validate sort column to prevent SQL injection
-        $allowedColumns = ['ticket_id', 'title', 'status', 'priority', 'category', 'type', 'created_at', 'updated_at'];
+        $allowedColumns = ['ticket_id', 'title', 'status', 'priority', 'category', 'type', 'created_at', 'updated_at', 'created_by', 'assigned_to'];
         if (!in_array($sortColumn, $allowedColumns)) {
             $sortColumn = 'ticket_id';
         }
-        
+
+        // Map column names to actual sort expressions
+        // For user columns, sort by display name with NULL handling for unassigned
+        $sortExpression = $sortColumn;
+        if ($sortColumn === 'created_by') {
+            $sortExpression = "COALESCE(u_created.display_name, u_created.username, 'System')";
+        } elseif ($sortColumn === 'assigned_to') {
+            // Put unassigned (NULL) at the end regardless of sort direction
+            $sortExpression = "CASE WHEN t.assigned_to IS NULL THEN 1 ELSE 0 END, COALESCE(u_assigned.display_name, u_assigned.username)";
+        } else {
+            $sortExpression = "t.$sortColumn";
+        }
+
         // Validate sort direction
         $sortDirection = strtolower($sortDirection) === 'asc' ? 'ASC' : 'DESC';
-        
+
         // Get total count for pagination
         $countSql = "SELECT COUNT(*) as total FROM tickets $whereClause";
         $countStmt = $this->conn->prepare($countSql);
-        
+
         if (!empty($params)) {
             $countStmt->bind_param($paramTypes, ...$params);
         }
-        
+
         $countStmt->execute();
         $totalResult = $countStmt->get_result();
         $totalTickets = $totalResult->fetch_assoc()['total'];
-        
+
         // Get tickets with pagination and creator info
         $sql = "SELECT t.*,
                 u_created.username as creator_username,
@@ -181,7 +193,7 @@ class TicketModel {
                 LEFT JOIN users u_created ON t.created_by = u_created.user_id
                 LEFT JOIN users u_assigned ON t.assigned_to = u_assigned.user_id
                 $whereClause
-                ORDER BY $sortColumn $sortDirection
+                ORDER BY $sortExpression $sortDirection
                 LIMIT ? OFFSET ?";
         $stmt = $this->conn->prepare($sql);