Conditional lead/lag function PostgreSQL?

Question

I have a table like this:

Name   activity  time

user1  A1        12:00
user1  E3        12:01
user1  A2        12:02
user2  A1        10:05
user2  A2                


        

Answer 1

select      distinct on(name) name,activity,next_activity

from       (select name,activity,time
                  ,lead(activity) over (partition by name order by time) as next_activity

            from   t

            where  left(activity,1) in ('A','B')
            ) t

where       left(activity,1) = 'A'

order by    name,time desc