Merge branch 'MDL-30734_21' of git://github.com/timhunt/moodle into MOODLE_21_STABLE

2011-12-19 11:52:17 +13:00
parent 0006e0cfa1 ba9cb0848a
commit dd98ec697c
1 changed files with 5 additions and 1 deletions
@@ -851,7 +851,11 @@ ORDER BY
     * @return string SQL code for the subquery.
     */
    public function sum_usage_marks_subquery($qubaid) {
-        return "SELECT SUM(qa.maxmark * qas.fraction)
+        // To explain the COALESCE in the following SQL: SUM(lots of NULLs) gives
+        // NULL, while SUM(one 0.0 and lots of NULLS) gives 0.0. We don't want that.
+        // We always want to return a number, so the COALESCE is there to turn the
+        // NULL total into a 0.
+        return "SELECT COALESCE(SUM(qa.maxmark * qas.fraction), 0)
            FROM {question_attempts} qa
            JOIN {question_attempt_steps} qas ON qas.id = (
                SELECT MAX(summarks_qas.id)