Use previous row result when current row is null - mysql

Question

I have a table that has daily records of transactions and some rows are missing data which will make plotting a daily graph inconsistent.
I want a query to use the last row result when the current one is null so that it can look something like this:
The structure of my table looks like this:
I have tried working on this query to select the previous row and update the current row if it is null but is not dynamic.
SELECT BALANCE
FROM tbl_batch_balances_null
WHERE id =
(select min(id)
from tbl_batch_balances_null where id < '2' and balance is not null)

Answer 1

Schema (MySQL v5.7)
DROP TABLE IF EXISTS alfie;
CREATE TABLE alfie
(id INT AUTO_INCREMENT PRIMARY KEY,
store CHAR(1) NULL,
product INT NULL
);
INSERT INTO alfie VALUES
( 7,'a',3),
( 8,null,null),
( 9,null,null),
(10,null,null),
(11,'a',1),
(12,'a',1),
(13,'a',1),
(14,null,null),
(15,null,null),
(16,'b',2),
(17,null,null),
(18,null,null),
(19,null,null);
Query #1
SELECT a.id,
COALESCE(a.store, c.store) store,
COALESCE(a.product,c.product) product
FROM alfie a
LEFT
JOIN
( SELECT x.*,
MAX(y.id) y_id
FROM alfie x
JOIN alfie y
ON y.id < x.id
AND y.store IS NOT NULL
WHERE x.store IS NULL
GROUP
BY x.id
) b
ON b.id = a.id
LEFT
JOIN alfie c
ON c.id = b.y_id
ORDER
BY id;
id
store
product
7
a
3
8
a
3
9
a
3
10
a
3
11
a
1
12
a
1
13
a
1
14
a
1
15
a
1
16
b
2
17
b
2
18
b
2
19
b
2
View on DB Fiddle