MySQL Finding the next row value by ID - mysql

Question

id month status
1 1997-11-01 A
1 2015-08-01 B
2 2010-01-01 A
2 2010-02-01 B
2 2012-10-01 C
That I would like to format to be:
id month lead_month status
1 1997-11-01 2015-08-01 A
1 2015-08-01 NOW() B
2 2010-01-01 2010-02-01 A
2 2010-02-01 2012-10-01 B
2 2012-10-01 NOW() C
MySQL is new to me, and I have trouble wrapping my head around variables. I would prefer to use a simple LEAD() with a PARTITION but unfortunately, I can't.
Here's my attempt, that doesn't work:
SET #lead = '1995-01-01'; --abitrary floor
select id, month, status, #lead, #lead:=month from table
The output looks like this, which also doesn't check if the id's from row to row are the same:
id month lead_month status
1 1997-11-01 1995-01-01 A
1 2015-08-01 1997-11-01 B
2 2010-01-01 2015-08-01 A
2 2010-02-01 2010-01-01 B
2 2012-10-01 2010-02-01 C

Answer 1

Don't muck around with variables in MySQL. That sort of logic would better reside in whatever language you are using for your application. This, however can be done in SQL.
My first instinct is simply to save that data in an extra column. Don't worry about the size of the db–there aren't enough months in the universe to become a problem.
There is also something wrong with your ids: these should almost always be primary keys, i. e. unique.
If you insist on your scheme, you can use a join. Assuming consecutive unique ids:
SELECT a.id, a.month, b.month AS lead_month, status FROM table AS a LEFT JOIN table AS b WHERE a.id - 1 = b.id;

Answer 2

You can use a correlated subquery:
select t.*,
(select t2.month
from t t2
where t.id = t2.id
order by t2.month desc
limit 1
) as next_month
from t;
If you want to replace the value for the last month for each id, then you can use coalesce():
select t.*,
coalesce((select t2.month
from t t2
where t.id = t2.id
order by t2.month desc
limit 1
), now()) as next_month
from t;