SQLite - GROUP BY rows, only when certain columns match another - sql

Question

I am building a schedule app, which uses a predefined API from my school. Each lesson has it's times declared for each hour in a single row. A short example:
Table lessons:
Subject Id Start End
English 111 09:30 10:30
English 111 10:30 11:30
Dutch 120 12:30 13:30
Java 109 14:30 15:30
English 111 15:30 16:30
To retrieve the lessons, and to show them inside my app, I use the following query:
SELECT MIN(start), MAX(end) FROM lessons ORDER BY Start GROUP BY Id
It works pretty well, however when a student has the same two lessons with the same Id on a day, in this case English, the SQL query will show it as:
English 09:30 - 16:30
In this case we have a problem, since English doesn't take 6 hours, and I want to show it as:
English 09:30 - 11:30
English 15:30 - 16:30
So my question is:
What query should I use to only GROUP BY lessons when the start or end values equals another row with the same id, to avoid wrong times?, I could do this programmaticly but I really prefer to do this using SQL.
EDIT:
I am grouping by, because I don't want to show each lesson as a seperate row, because my school defines every lessonhour as a single row.

Answer 1

This will do the job
with cte as
(
select subject,id,start,end
from mytable t
where not exists
(
select null
from mytable t2
where t2.id = t.id
and t2.end = t.start
)
union all
select t.subject,t.id,cte.start,t.end
from cte
join mytable t
on t.id = cte.id
and t.start = cte.end
)
select subject,id,start,max(end) as end
from cte
group by subject,id,start
+---------+-----+-------+-------+
| subject | id | start | end |
+---------+-----+-------+-------+
| Dutch | 120 | 12:30 | 13:30 |
+---------+-----+-------+-------+
| English | 111 | 09:30 | 11:30 |
+---------+-----+-------+-------+
| English | 111 | 15:30 | 16:30 |
+---------+-----+-------+-------+
| Java | 109 | 14:30 | 15:30 |
+---------+-----+-------+-------+