Most efficient way to sum overlapping datetimes in MySQL - mysql

Question

I'm in the process of evaluating the proposed solutions on SO related to the sum of overlapping datetimes in MySQL. I wasn't able to find out a silver-bullet solution, so would like to know if any classic/industrial-grade algorithmic procedure is available or if a custom-made needs to be developed.
Total should be 8 hours (4+4).
Proposed solution through MySQL
function final_balance($teacher_id, $aa, $teaching_id=0) {
$dbo = $this->Attendance->getDataSource();
$years=$this->Attendance->Student->Year->find('list', array('fields' => array('anno', 'data_from')));
$filteraa='attendances.start>="'.$years[$aa].'"';
$this->query('SET #interval_id = 0');
$this->query('SET #interval_end = \'1970-01-01 00:00:00\'');
$sql='SELECT
MIN(start) as start,
MAX(end) as end
FROM (
SELECT
#interval_id := IF(attendances.start > #interval_end, #interval_id + 1, #interval_id) AS interval_id,
#interval_end := IF(attendances.start < #interval_end, GREATEST(#interval_end, attendances.end), attendances.end) AS interval_end,
attendances.start,
attendances.end
FROM attendances
INNER JOIN attendance_sheets ON (
attendance_sheet_id = attendance_sheets.id AND
attendance_sheets.teacher_id='.$teacher_id.' AND '.$filteraa.' AND
attendance_sheet_status_id = 2 AND
attendance_status_id!=3'.
($teaching_id?' AND attendances.teaching_id IN ('.$teaching_id.')':'').'
)
ORDER BY attendances.start,attendances.end
) intervals GROUP BY interval_id';
// final query to sum in the temp table
$finalStatement =array(
'table' => $dbo->expression('('.$sql.')')->value,
'alias' => 'Attendance',
'fields' => array(
'DATE_FORMAT(start, \'%d/%m/%Y\') as data',
'DATE_FORMAT(start, \'%m-%Y\') as datamese',
'DATE(start) as datasql',
$teacher_id.' AS teacher_id',
'DAY(start) as giorno',
'MONTH(start) as mese',
'YEAR(start) as anno',
'SEC_TO_TIME(SUM((TIME_TO_SEC(end) - TIME_TO_SEC(start)))) as ore',
),
'conditions' => array(),
'limit' => null,
'group' => array('CONCAT(YEAR(start),MONTH(start))', 'DATE(start) WITH ROLLUP'),
'order' => null
);
$finalQuery= $dbo->buildStatement($finalStatement, $this->Attendance);
return $this->Attendance->query($dbo->expression($finalQuery)->value);
}
References
Sum amount of overlapping datetime ranges in MySQL
performs a different task
MySQL: sum time ranges exluding overlapping ones
and
MySQL: sum datetimes without counting overlapping periods twice
both seems to me like not considering all the cases
GeeksForCode: Merge Overlapping Intervals

Answer 1

Depending on the circumstances, the following might be useful and efficient.
Create another table that has one row per hour. Inner join that table with your table while selecting only the new column and dedupe the rows.
You can keep increasing the resolution (eg. to minutes or seconds), but that might make your code run slow.