diff --git a/course/classes/reportbuilder/local/entities/completion.php b/course/classes/reportbuilder/local/entities/completion.php
index e3fa21df4ce..69b0db9edf5 100644
--- a/course/classes/reportbuilder/local/entities/completion.php
+++ b/course/classes/reportbuilder/local/entities/completion.php
@@ -107,15 +107,14 @@ class completion extends base {
         ))
             ->add_joins($this->get_joins())
             ->set_type(column::TYPE_BOOLEAN)
-            ->add_field("CASE WHEN {$coursecompletion}.timecompleted > 0 THEN 1 ELSE 0 END", 'completed')
-            ->add_field("{$user}.id", 'userid')
+            ->add_field("
+                CASE
+                    WHEN {$coursecompletion}.id IS NULL THEN NULL
+                    WHEN {$coursecompletion}.timecompleted > 0 THEN 1
+                    ELSE 0
+                END", 'completed')
             ->set_is_sortable(true)
-            ->add_callback(static function(bool $value, stdClass $row): string {
-                if (!$row->userid) {
-                    return '';
-                }
-                return format::boolean_as_text($value);
-            });
+            ->add_callback([format::class, 'boolean_as_text']);
 
         // Completion criteria column.
         $criterias = database::generate_alias();
@@ -232,14 +231,14 @@ class completion extends base {
             ->set_type(column::TYPE_INTEGER)
             ->add_field("(
                 CASE
-                    WHEN {$coursecompletion}.timecompleted > 0 THEN
+                    WHEN {$coursecompletion}.id IS NULL THEN NULL
+                    ELSE (CASE WHEN {$coursecompletion}.timecompleted > 0 THEN
                         {$coursecompletion}.timecompleted
-                    ELSE
+                        ELSE
                         {$currenttime}
-                END - {$course}.startdate) / " . DAYSECS, 'dayscourse')
-            ->add_field("{$user}.id", 'userid')
-            ->set_is_sortable(true)
-            ->add_callback([completion_formatter::class, 'get_days']);
+                    END - {$course}.startdate) / " . DAYSECS . "
+                END)", 'dayscourse')
+            ->set_is_sortable(true);
 
         // Days since last completion (days since last enrolment date until completion or until current date if not completed).
         $columns[] = (new column(
@@ -251,14 +250,14 @@ class completion extends base {
             ->set_type(column::TYPE_INTEGER)
             ->add_field("(
                 CASE
-                    WHEN {$coursecompletion}.timecompleted > 0 THEN
+                    WHEN {$coursecompletion}.id IS NULL THEN NULL
+                    ELSE (CASE WHEN {$coursecompletion}.timecompleted > 0 THEN
                         {$coursecompletion}.timecompleted
-                    ELSE
+                        ELSE
                         {$currenttime}
-                END - {$coursecompletion}.timeenrolled) / " . DAYSECS, 'daysuntilcompletion')
-            ->add_field("{$user}.id", 'userid')
-            ->set_is_sortable(true)
-            ->add_callback([completion_formatter::class, 'get_days']);
+                    END - {$coursecompletion}.timeenrolled) / " . DAYSECS . "
+                END)", 'daysuntilcompletion')
+            ->set_is_sortable(true);
 
         // Student course grade.
         $columns[] = (new column(
diff --git a/course/classes/reportbuilder/local/formatters/completion.php b/course/classes/reportbuilder/local/formatters/completion.php
index c031b4e4e8f..786b04d05f2 100644
--- a/course/classes/reportbuilder/local/formatters/completion.php
+++ b/course/classes/reportbuilder/local/formatters/completion.php
@@ -66,8 +66,13 @@ class completion {
      * @param int|null $value
      * @param stdClass $row
      * @return int|null
+     *
+     * @deprecated since Moodle 4.5 - please do not use this function any more
      */
+    #[\core\attribute\deprecated(null, mdl: 'MDL-82467', since: '4.5')]
     public static function get_days(?int $value, stdClass $row): ?int {
+        \core\deprecation::emit_deprecation_if_present([self::class, __FUNCTION__]);
+
         // Do not show anything if there is no userid.
         if (!$row->userid) {
             return null;
diff --git a/course/tests/reportbuilder/datasource/participants_test.php b/course/tests/reportbuilder/datasource/participants_test.php
index e3c8a4e45b8..b4d4ff9eb39 100644
--- a/course/tests/reportbuilder/datasource/participants_test.php
+++ b/course/tests/reportbuilder/datasource/participants_test.php
@@ -243,7 +243,7 @@ final class participants_test extends core_reportbuilder_testcase {
 
 
     /**
-     * Test creating course report, with aggregated last access date (minimum and maximum)
+     * Test creating participants report, with aggregated last access date (minimum and maximum)
      */
     public function test_course_last_access_aggregation(): void {
         $this->resetAfterTest();
@@ -259,7 +259,7 @@ final class participants_test extends core_reportbuilder_testcase {
         /** @var core_reportbuilder_generator $generator */
         $generator = $this->getDataGenerator()->get_plugin_generator('core_reportbuilder');
 
-        $report = $generator->create_report(['name' => 'Courses', 'source' => participants::class, 'default' => 0]);
+        $report = $generator->create_report(['name' => 'Participants', 'source' => participants::class, 'default' => 0]);
         $generator->create_column(['reportid' => $report->get('id'), 'uniqueidentifier' => 'course:fullname']);
         $column = $generator->create_column(['reportid' => $report->get('id'), 'uniqueidentifier' => 'access:timeaccess']);
 
@@ -278,6 +278,43 @@ final class participants_test extends core_reportbuilder_testcase {
         ], array_map('array_values', $content));
     }
 
+    /**
+     * Test creating participants report, with aggregated days taking course column
+     */
+    public function test_completion_days_taking_course_aggregation(): void {
+        $this->resetAfterTest();
+
+        $courseone = $this->getDataGenerator()->create_course(['fullname' => 'Course 1', 'startdate' => 1622502000]);
+        $coursetwo = $this->getDataGenerator()->create_course(['fullname' => 'Course 2']);
+
+        // User one completed the course in two days.
+        $userone = $this->getDataGenerator()->create_and_enrol($courseone);
+        $completion = new completion_completion(['course' => $courseone->id, 'userid' => $userone->id]);
+        $completion->mark_complete(1622502000 + (2 * DAYSECS));
+
+        // User two completed the course in three days (lazy bum).
+        $usertwo = $this->getDataGenerator()->create_and_enrol($courseone);
+        $completion = new completion_completion(['course' => $courseone->id, 'userid' => $usertwo->id]);
+        $completion->mark_complete(1622502000 + (3 * DAYSECS));
+
+        /** @var core_reportbuilder_generator $generator */
+        $generator = $this->getDataGenerator()->get_plugin_generator('core_reportbuilder');
+
+        $report = $generator->create_report(['name' => 'Participants', 'source' => participants::class, 'default' => 0]);
+        $generator->create_column(['reportid' => $report->get('id'), 'uniqueidentifier' => 'course:fullname', 'sortenabled' => 1]);
+        $generator->create_column([
+            'reportid' => $report->get('id'),
+            'uniqueidentifier' => 'completion:dayscourse',
+            'aggregation' => 'avg',
+        ]);
+
+        $content = $this->get_custom_report_content($report->get('id'));
+        $this->assertEquals([
+            [$courseone->fullname, '2.5'],
+            [$coursetwo->fullname, ''],
+        ], array_map('array_values', $content));
+    }
+
     /**
      * Data provider for {@see test_datasource_filters}
      *