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MDL-30734 question engine: sum_usage_marks_subquery edge-case.

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When all qas in a useage are 'gaveup' state, it gives NULL, not 0.0, for the total.
This commit is contained in:
Tim Hunt 2011-12-14 14:29:00 +00:00
parent f89a83b87b
commit ad73a2cb4e
1 changed files with 5 additions and 1 deletions
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6
question/engine/datalib.php
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@ -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)