MySQL 8.0

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create table lms_leads(id int, status varchar(10), assigned_to varchar(10), date datetime);
insert into lms_leads values
(1, 'open', 'Smith', '2018-08-26'),
(2, 'open', 'Drew', '2018-08-26'),
(3, 'new', 'Amit', '2018-08-26')
create table lms_leads_comments(id int, lead_id int, comment_data varchar(130), follow_up_time datetime);
insert into lms_leads_comments values
(1, 1, 'old task line2', '2018-08-27 14:18:26'),
(2, 2, 'old task line1', '2018-08-27 14:18:26'),
(3, 1, 'new task line1', '2018-08-27 17:18:00'),
(4, 2, 'new task line3', '2018-09-27 20:18:26'),
(5, 2, 'old task line2', '2018-08-27 21:18:26');
 select @rn := 1, @lead_id_lag := 0; 


@rn := 1
@lead_id_lag := 0


1
0


select l.id lead_id, 
       lc.id comment_id,
       lc.follow_up_time,
       lc.comment_data,
       l.assigned_to
from lms_leads l
left join (
    select case when @lead_id_lag = lead_id then @rn := @rn + 1 else @rn := 1 end rn,
           @lead_id_lag := lead_id,
           id,
           follow_up_time,
           comment_data,
           lead_id
    from lms_leads_comments
    order by lead_id, follow_up_time desc
) lc on l.id = lc.lead_id and lc.rn = 1;


lead_id
comment_id
follow_up_time
comment_data
assigned_to


1
3
2018-08-27 17:18:00
new task line1
Smith

2
4
2018-09-27 20:18:26
new task line3
Drew

3
null
null
null
Amit

lead_id	comment_id	follow_up_time	comment_data	assigned_to
1	3	2018-08-27 17:18:00	new task line1	Smith
2	4	2018-09-27 20:18:26	new task line3	Drew
3	null	null	null	Amit