Update column with value from last value until next populated value - mysql

Question

I have a table which contains an amount per month and a previous month amount.
Each month I need to carry the last previous month amount if it does not exist.
To explain slightly better (and with examples) I might have the following data;
Month,Amount,Previous
2019-01-01,100,0
2019-02-01,100,100
2019-03-01,100,null
2019-04-01,100,null
2019-05-01,100,200
2019-06-01,100,null
So I want to carry the 100 to March and April and then the 200 to June so it looks like this;
Month,Amount,Previous
2019-01-01,100,0
2019-02-01,100,100
2019-03-01,100,100
2019-04-01,100,100
2019-05-01,100,200
2019-06-01,100,200
I'm just hitting blanks, I know there is a way but the mind simply isn't putting it together.
I think it's going to involve LEFT JOIN on the same table and getting a MIN month value with the amount where the date is greater than the last month value but is not greater than the next.
Or it's going to be doing a subquery in a WHERE clause and a LEFT JOIN.
So far I've managed the below but it duplicates the May and June rows for each previous value (100 and 200).
SELECT
*
FROM
table1 t1
LEFT JOIN
table1 t2 ON
t1.month > t2.month
Month,Amount,Previous
2019-01-01,100,0
2019-02-01,100,100
2019-03-01,100,100
2019-04-01,100,100
2019-05-01,100,200
2019-05-01,100,100
2019-06-01,100,200
2019-06-01,100,100

Answer 1

drop table if exists t;
create table t
(Month date,Amount int,Previous int);
insert into t values
('2019-01-01',100,0),
('2019-02-01',100,100),
('2019-03-01',100,null),
('2019-04-01',100,null),
('2019-05-01',100,200),
('2019-06-01',100,null);
select t.*,
case when previous is null then
(select previous from t t1 where t1.month < t.month and t1.previous is not null order by month desc limit 1)
else previous
end as previousdownfilled
from t;
+------------+--------+----------+--------------------+
| Month | Amount | Previous | previousdownfilled |
+------------+--------+----------+--------------------+
| 2019-01-01 | 100 | 0 | 0 |
| 2019-02-01 | 100 | 100 | 100 |
| 2019-03-01 | 100 | NULL | 100 |
| 2019-04-01 | 100 | NULL | 100 |
| 2019-05-01 | 100 | 200 | 200 |
| 2019-06-01 | 100 | NULL | 200 |
+------------+--------+----------+--------------------+
6 rows in set (0.00 sec)
where the case statement checks if things need to be done and the correlated cub query does it. But I suspect this work should be done in the code which creates this table.

Answer 2

You can use a Correlated Subquery here. In the subquery, we will utilize ORDER BY with LIMIT to get the immediate previous amount value.
SELECT
t1.month,
t1.amount,
(SELECT t2.amount FROM table1 t2
WHERE t2.month < t1.month
ORDER BY t2.month DESC LIMIT 1) AS previous
FROM
table1 t1

Answer 3

In MySL 8+, you can use window functions. Unfortunately, MySQL has not (yet) implemented the simplest approach -- lag() with the ignore nulls option.
But you can still do this pretty simply:
select month, amount,
max(previous) over (partition by grp) as previous
from (select t.*, count(previous) over (order by month) as grp
from t
) t;