diff --git a/privacy/classes/local/request/contextlist.php b/privacy/classes/local/request/contextlist.php
index 4e391abcb75..b9b2cd09cf7 100644
--- a/privacy/classes/local/request/contextlist.php
+++ b/privacy/classes/local/request/contextlist.php
@@ -138,6 +138,12 @@ class contextlist extends contextlist_base {
      * @return  string  The field name or a numeric value representing the context id
      */
     protected function guess_id_field_from_sql(string $sql) : string {
+        // We are not interested in any subquery/view/conditions for the purpose of this method, so
+        // let's reduce the query to the interesting parts by recursively cleaning all
+        // contents within parenthesis. If there are problems (null), we keep the text unmodified.
+        // So just top-level sql will remain after the reduction.
+        $recursiveregexp = '/\((([^()]*|(?R))*)\)/';
+        $sql = (preg_replace($recursiveregexp, '', $sql) ?: $sql);
         // Get the list of relevant words from the SQL Query.
         // We explode the SQL by the space character, then trim any extra whitespace (e.g. newlines), before we filter
         // empty value, and finally we re-index the array.
@@ -147,8 +153,15 @@ class contextlist extends contextlist_base {
             return $word !== '';
         });
         $words = array_values($words);
+        $uwords = array_map('strtoupper', $words); // Uppercase all them.
 
-        if ($firstfrom = array_search('FROM', array_map('strtoupper', $words))) {
+        // If the query has boolean operators (UNION, it is the only one we support cross-db)
+        // then we cannot guarantee whats coming after the first query, it can be anything.
+        if (array_search('UNION', $uwords)) {
+            return '';
+        }
+
+        if ($firstfrom = array_search('FROM', $uwords)) {
             // Found a FROM keyword.
             // Select the previous word.
             $fieldname = $words[$firstfrom - 1];
diff --git a/privacy/tests/contextlist_test.php b/privacy/tests/contextlist_test.php
index a247b3d3ab9..43d8b74ea9d 100644
--- a/privacy/tests/contextlist_test.php
+++ b/privacy/tests/contextlist_test.php
@@ -176,6 +176,27 @@ class contextlist_test extends advanced_testcase {
                 'SELECT 4 FROM users',
                 '4',
             ],
+            'select_with_complex_subqueries' => [
+                'SELECT id FROM table WHERE id IN (
+                     SELECT x FROM xtable
+                     UNION
+                     SELECT y FROM (
+                         SELECT y FROM ytable
+                         JOIN ztable ON (z = y)))',
+                'id'
+            ],
+            'invalid_union_with_first_being_column_name' => [
+                'SELECT id FROM table UNION SELECT 1 FROM table',
+                ''
+            ],
+            'invalid_union_with_first_being_numeric' => [
+                'SELECT 1 FROM table UNION SELECT id FROM table',
+                ''
+            ],
+            'invalid_union_without_from' => [
+                'SELECT 1 UNION SELECT id FROM table',
+                ''
+            ],
             'invalid_1' => [
                 'SELECT 1+1',
                 '',