mysql - Simplify the timeline in SQL(Netezza) -

i want summarize/simplify(i don't know call) timeline.

so have id timeline. trying rid of overlap timeline within same id.

here example of data. have:

id   start_time   end_time  1                 b  1        c          d  1        e          f  1        g          h 

can see picture, [a,b],[c,d],[e,f] overlap each other , [g,h] disjoint want [a,f] , [g,h]. want:

id   start_time   end_time  1                 f  1        g          h 

i think @sha.t close. problem in multiple overlaps break down. might have turn multiple step process

step 1 (make sample table):

 create temp table stack (  id integer  ,start_time timestamp  ,end_time timestamp  )   insert stack  values(1, date('2020-01-01'),date('2020-01-01') + interval '3 hours');  insert stack  values(1,date('2020-01-01') + interval '2 hours',date('2020-01-01') + interval '4 hours');  insert stack  values(1,date('2020-01-01') + interval '3.5 hours',date('2020-01-01') + interval '5 hours');  insert stack  values(1,date('2020-01-01') + interval '5.5 hours',date('2020-01-01') + interval '6.5 hours');  insert stack  values(1,date('2020-01-01') + interval '7.5 hours',date('2020-01-01') + interval '9.5 hours');  insert stack  values(1,date('2020-01-01') + interval '8.5 hours',date('2020-01-01') + interval '10.5 hours'); 

step 2 (find single overlaps):

create temp table stack2  select id, ps2 start_time, max(e) end_time     (         select t1.id, t1.start_time s, max(t1.end_time) e,                max(t2.start_time) ps, max(t2.end_time) pe                ,case when pe between s , e ps else s end ps2         stack t1         join stack t2 on t1.start_time > t2.start_time         group t1.id, t1.start_time) dt     group         id, ps2     order ps2 

step 3 (consolidate double overlaps):

 select id, ps2 start_time, max(e) end_time     (         select t1.id, t1.start_time s, max(t1.end_time) e,                max(t2.start_time) ps, max(t2.end_time) pe                ,case when pe between s , e ps else s end ps2         stack2 t1         join stack2 t2 on t1.start_time > t2.start_time         group t1.id, t1.start_time) dt     group         id, ps2     order ps2