How to add hours matching a mysql query? - mysql

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.

Answer 1

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 |
+---------+---------------+

Answer 2

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'])).