Complex queries to find sum of rows depending on date - mysql

Question

CREATE TABLE master_tb
(
date DATE
item VARCHAR(10)
change INT
current INT
);
INSERT INTO master_tb (date, item, change, current)
VALUES
("2021-01-01", "ABC", 11, 11),
("2021-01-01", "KLM", 4, 4),
("2021-01-02", "KLM", -3, 1),
("2021-01-03", "KLM", -1, 0),
("2021-02-01", "KLM", 6, 6),
("2021-02-02", "XYZ", 5, 5),
("2021-02-08", "KLM", -3, 3),
("2021-02-09", "XYZ", -1, 4),
("2021-03-02", "XYZ", 2, 6),
("2021-03-08", "XYZ", -1, 5),
("2021-03-08", "KLM", -3, 0);
I have the above table for an inventory log. I want to get 2 things:
The current value given a #date. So if my given date is 2021-03-09, even though that date doesn't exist in the list, it will give me the most recent values of all ABC, XYZ, and KLM items and their current status. So the select table would look something like this:
+------+---------+
| item | current |
+======+=========+
| ABC | 11 |
+------+---------+
| XYZ | 5 |
+------+---------+
| KLM | 0 |
+------+---------+
Similarly, I want to get the current values but for specific timeframes, given a "now" date #date where that can be any date. So If #date = 2021-4-1, I am looking for something like this:
+------+-------+-----------+------------+------------+----------+
| item | total | 0-30 days | 31-60 days | 61-90 days | 90+ days |
+======+=======+===========+============+============+==========+
| XYZ | 5 | 1 | 4 | 0 | 0 |
+------+-------+-----------+------------+------------+----------+
| ABC | 11 | 0 | 0 | 11 | 0 |
+------+-------+-----------+------------+------------+----------+
| KLM | 0 | 0 | 0 | 0 | 0 |
+------+-------+-----------+------------+------------+----------+
One thing to note is that "older" items are deducted if there was a deduction. So if 5x item XYZ was added 50 days ago, and 2x 20 days ago, and it was reduced 3x 10 days ago, the table would show total = 4, 0-30 days = 2, and 31-60 days = 2 because the older items were deducted even tho the deduction occurred recently.
My first guess was to utilize partitions but I am not sure if that's possible, knowing the values outside of a partition is affected.
EDIT:
After a friend pointed out this article I have found a way to answer the first part of the question:
SELECT
item,
MAX(curr) as current // needed to group item columns
FROM(
SELECT
*,
LAST_VALUE(current) OVER // partitioning was needed to
( // find the most recent 'current'
PARTITION BY item // value added on that 'item'
ORDER BY date
RANGE BETWEEN
UNBOUNDED PRECEDING AND
UNBOUNDED FOLLOWING
) curr
FROM tbl
WHERE date <= #date
)a
WHERE bal = balance
GROUP BY item
;

Answer 1

To answer the first part of your question, consider the following...
CREATE TABLE master_tb
( date DATE
, item VARCHAR(10)
, change_x INT
, current INT
, PRIMARY KEY(date,item)
);
INSERT INTO master_tb (date, item, change_x, current)
VALUES
("2021-01-01", "ABC", 11, 11),
("2021-01-01", "KLM", 4, 4),
("2021-01-02", "KLM", -3, 1),
("2021-01-03", "KLM", -1, 0),
("2021-02-01", "KLM", 6, 6),
("2021-02-02", "XYZ", 5, 5),
("2021-02-08", "KLM", -3, 3),
("2021-02-09", "XYZ", -1, 4),
("2021-03-02", "XYZ", 2, 6),
("2021-03-08", "XYZ", -1, 5),
("2021-03-08", "KLM", -3, 0);
SELECT item
, MAX(date) date
FROM master_tb
WHERE date <= '2021-03-09'
GROUP
BY item;
+------+------------+
| item | date |
+------+------------+
| ABC | 2021-01-01 |
| KLM | 2021-03-08 |
| XYZ | 2021-03-08 |
+------+------------+
SELECT a.*
FROM master_tb a
JOIN
( SELECT item
, MAX(date) date
FROM master_tb
WHERE date <= '2021-03-09'
GROUP
BY item
) b
ON b.item = a.item
AND b.date = a.date;
+------------+------+----------+---------+
| date | item | change_x | current |
+------------+------+----------+---------+
| 2021-01-01 | ABC | 11 | 11 |
| 2021-03-08 | KLM | -3 | 0 |
| 2021-03-08 | XYZ | -1 | 5 |
+------------+------+----------+---------+
We could also solve this using common table expressions/windowing functions, and that might be useful when we come to look at the next part of the problem - although I have a suspicion that much of that should be resolved in application code.