MariaDB 10.3

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CREATE TABLE inventory (
    inbound_date DATE,
    product_name VARCHAR(255)
);

INSERT INTO inventory
(inbound_date, product_name)
VALUES 
("2020-01-25", "Product_A"),
("2020-02-18", "Product_B"),
("2020-03-14", "Product_C");

Records: 3  Duplicates: 0  Warnings: 0


SET
@query_date = '2020-04-02';

SELECT
product_name,
@query_date,
TIMESTAMPDIFF(DAY, MAX(inbound_date), @query_date) AS days_in_inventory
FROM inventory
GROUP BY 1;


product_name
@query_date
days_in_inventory


Product_A
2020-04-02
68

Product_B
2020-04-02
44

Product_C
2020-04-02
19


with recursive dates as (
      select date('2020-04-01') as date 
      union all
      select date + interval 1 day
      from dates
      where date < '2020-04-04'
     )
SELECT product_name, d.date,
       TIMESTAMPDIFF(DAY, i.inbound_date, d.date) AS days_in_inventory
FROM dates d CROSS JOIN
     inventory i
ORDER By d.date, i.product_name


product_name
date
days_in_inventory


Product_A
2020-04-01
67

Product_B
2020-04-01
43

Product_C
2020-04-01
18

Product_A
2020-04-02
68

Product_B
2020-04-02
44

Product_C
2020-04-02
19

Product_A
2020-04-03
69

Product_B
2020-04-03
45

Product_C
2020-04-03
20

Product_A
2020-04-04
70

Product_B
2020-04-04
46

Product_C
2020-04-04
21

product_name	@query_date	days_in_inventory
Product_A	2020-04-02	68
Product_B	2020-04-02	44
Product_C	2020-04-02	19

product_name	date	days_in_inventory
Product_A	2020-04-01	67
Product_B	2020-04-01	43
Product_C	2020-04-01	18
Product_A	2020-04-02	68
Product_B	2020-04-02	44
Product_C	2020-04-02	19
Product_A	2020-04-03	69
Product_B	2020-04-03	45
Product_C	2020-04-03	20
Product_A	2020-04-04	70
Product_B	2020-04-04	46
Product_C	2020-04-04	21