How to add hours matching a mysql query?

Question

Data (Table name: T1):

Teacher	Subject	Day	Hour
Albert	Blue	Wednesday	10:00
Albert	Blue	Wednesday	12:00
Brandon	Red	Tuesday	09:00
Brandon	Red	Tuesday	11:00
Albert	Cyan	Monday	08:30
Albert	Cyan	Monday	10:30
Claudia	Gray	Thursday	08:00
Claudia	Gray	Thursday	10:00
Albert	Pink	Friday	13:00
Albert	Pink	Friday	14:30
Martha	Green	Wednesday	12:00
Martha	Green	Wednesday	14:00
Albert	Yellow	Friday	11:00
Albert	Yellow	Friday	12:30

As it can be seen, there is a record for the starting hour of a Subject and another for the finishing time of the same Subject (data comes like that). What I intend to know is the weekly amount of hours dedicated to classes by a specific teacher, let's say for "Albert".

Query:

$result = mysqli_query($link, "SELECT SUBTIME(max(Hour), min(Hour)) AS TeachTime, Subject FROM T1 WHERE Teachers LIKE '%Albert%' GROUP BY Subject ORDER BY Subject ASC") or die(mysqli_error($link));
if (mysqli_num_rows($result)!==0) {
echo "Weekly teaching time: ";
while($row = mysqli_fetch_array($result)){
echo $row['TeachTime']." Hrs. ";
}
}

Output:

Weekly teaching time: 02:00 Hrs. 02:00 Hrs. 01:30 Hrs. 01:30 Hrs.

Desired output:

Weekly teaching time: 07:00 Hrs.

As you can see, I don't know how to perform the addition of every resulting amount of hours. How can I achieve that?

I have also tried GROUP BY Teachers but results are weird... not the addition result.

It is a very bad idea to use `die(mysqli_error($$conn));` in your code, because it could potentially leak sensitive information. See this post for more explanation: [mysqli or die, does it have to die?](https://stackoverflow.com/a/15320411/1839439) — Dharman, May 31 '22 at 15:01
"As it can be seen, there is a record for the starting hour of a Subject and another for the finishing time of the same Subject" I only see an `Hour` column, and some guessing is needed to know if it is a start, or and end, time. — Luuk, May 31 '22 at 15:20

score 2 · Answer 1 · answered May 31 '22 at 15:11

You know how to get this:

mysql> select teacher, day, min(hour), max(hour) 
from T1 where teacher = 'Albert' group by teacher, day;
+---------+-----------+-----------+-----------+
| teacher | day       | min(hour) | max(hour) |
+---------+-----------+-----------+-----------+
| Albert  | Wednesday | 10:00:00  | 12:00:00  |
| Albert  | Monday    | 08:30:00  | 10:30:00  |
| Albert  | Friday    | 11:00:00  | 14:30:00  |
+---------+-----------+-----------+-----------+

And you can use TIMESTAMPDIFF() to calculate the minutes:

mysql> select teacher, day, 
timestampdiff(minute, min(hour), max(hour)) as minutes 
from T1 where teacher = 'Albert' group by teacher, day;
+---------+-----------+---------+
| teacher | day       | minutes |
+---------+-----------+---------+
| Albert  | Wednesday |     120 |
| Albert  | Monday    |     120 |
| Albert  | Friday    |     210 |
+---------+-----------+---------+

Now wrap that as a subquery in another aggregation query:

mysql> select teacher, sum(minutes) as total_minutes 
from (
  select teacher, day, 
  timestampdiff(minute, min(hour), max(hour)) as minutes 
  from T1 where teacher = 'Albert' group by teacher, day) as t 
group by teacher;
+---------+---------------+
| teacher | total_minutes |
+---------+---------------+
| Albert  |           450 |
+---------+---------------+

Thanks a lot Bill, doing some little adjustments your solution gave me the expected results, I add the final code I'm using thanks to you. — Andrés Chandía, Jun 01 '22 at 14:51

Andrés Chandía · Accepted Answer · 2022-06-02T11:16:57.203

This is the code I end up using thanks to the guidance of Bill Karwin and some more research:

$result = mysqli_query($link, "SELECT *,
SEC_TO_TIME(SUM(TIME_TO_SEC(WeekTime))) AS ProfTime FROM
(SELECT *,
SUBTIME(max(Hour), min(Hour)) AS WeekTime FROM T1
WHERE Teacher LIKE '%Albert%' GROUP BY Teacher, Subject) AS C")
or die(mysqli_error($link));
if (mysqli_num_rows($result)!==0) {
echo "Weekly teaching time: ";
while($row = mysqli_fetch_array($result)){
echo date('H:i', strtotime($row['ProfTime']))." Hrs. <br>";
}}

Bill's answer had made me realize that you can query results from an embedded query, e.g.:

Query2 over results of (Query1 over data)

Then, as results of query1 were in time format (HH:MM), I should get a total of the addition of all the instances retrieved, i.e., 01:00 + 02:00 + 01:30 = 04:30

The addition has to be performed in query2 and I have found on tutorialspoint that the query SELECT SEC_TO_TIME(SUM(TIME_TO_SEC(Time))) AS TotalTime does that.

Finally, as the result comes in the the format HH:MM:SS and I only need HH:MM I applied a php convertion to the output: date('H:i', strtotime($row['TotalTime'])).

How to add hours matching a mysql query?

2 Answers2