How to replace a null value with previous non-null value? - mysql

Question

I want to replace a null value with previous non-null value using MySQL.
I tried this:
SELECT
`Date_Column`
,CASE
WHEN `Value_Column` is null
THEN (
SELECT
`Value_Column`
FROM
table_name t2
WHERE
`Date_Column` = (
SELECT
MAX(`Date_Column`)
FROM
table_name t3
WHERE
`Date_Column` < t1.`Date_Column`
AND `Value_Column` > 0
)
)
ELSE `Value_Column`
END AS `Value_Column`
FROM
table_name t1
This works but takes really long for big datasets.
I tried this for a subset of data and it worked.
Is there an easier/more efficient way to achieve the same?
Thanks.

Answer 1

Based on your query, your first check should be that your take is correctly indexed by the date column (and the value column as a covering index). If it is, then you could use the following slightly simplified query.
Note, this replaces NULL as per your description, where yours replaced 0 contrary to your description, you should be clear as to Exactly which behaviour you want.
SELECT
date_column,
COALESCE(
value_column,
(
SELECT lookup.value_column
FROM table_name AS lookup
WHERE lookup.date_column < table_name.date_column
AND lookup.value_column IS NOT NULL
AND table_name.value_column IS NULL
ORDER BY lookup.date_column DESC
LIMIT 1
)
)
FROM
table_name
(on my phone, so please forgive typos)

Answer 2

You can simplify the query to:
SELECT t1.Date_Column
(CASE WHEN t1.Value_Column = 0
THEN (SELECT t2.Value_Column
FROM table_name t2
WHERE t2.Date_Column < t1.Date_Column AND t2.Value_Column > 0
ORDER BY t2.Date_Column DESC
LIMIT 1
)
ELSE t2.Value_Column
END) AS Value_Column
FROM table_name t1;
This is an improvement on your query because it removes the second nested subquery. But it will still be slow. An index on table_name(Date_Column, Value_Column) might help.