Add months in db date using interval in mysql - mysql

Question

I want to add month in transaction date using mysql interval function by join plan table and transaction table,however this method not working but If I add months in static way to transaction date it is working.
plan table:
plan_id plan
1 6 month
2 12 month
3 3 month
transaction table:
id user_id subscribed_on plan_id
1 2 2020-04-04 1
2 4 2019-02-22 2
Mysql query (not working):
SELECT t.* FROM transaction t inner join plan p on p.plan_id=t.plan_id
where t.user_id=2 and DATE_ADD(date(t.subscribed_on), INTERVAL p.plan) >= CURDATE()
order by t.id desc
If I add month in static way than it is working fine:
SELECT t.* FROM transaction t inner join plan p on p.plan_id=t.plan_id
where t.user_id=2 and DATE_ADD(date(t.subscribed_on),
INTERVAL 6 month) >= CURDATE()
order by t.id desc

Answer 1

MySQL does not support using interval that way. Unlike in other databaes (such as Postgres for example), the unit argument is a keyword, not a literal string.
I would suspect that your table may store other intervals than just months (say, years, days, and so on). If so, you can use string functions and a case expression to accommodate the different possible values, like:
select t.*
from transaction t
inner join plan p on p.plan_id = t.plan_id
where
t.user_id = 2
and date(t.subscribed_on) + case substring_index(p.plan, ' ', -1)
when 'year' then interval substring_index(p.plan, ' ', 1) year
when 'month' then interval substring_index(p.plan, ' ', 1) month
when 'day' then interval substring_index(p.plan, ' ', 1) day
end
>= current_date
order by t.id desc
The logic here is to split the stored interval string into two parts: the number, and the unit; the case expression processes the unit and generate the proper literal interval accordingly.

Answer 2

Unfortunately a string in the data is not equivalent to an interval. One method is:
date(t.subscribed_on) + interval substring_index(plan, ' ') + 0 month
Note here that month is a keyword, not a string.

Answer 3

Try to force the plan column in the plan table to be an integer. Does not seem to be possible to cast a string to an interval.
I tried like so:
WITH
plan( plan_id,plan) AS (
SELECT 1,'6 month'
UNION ALL SELECT 2,'12 month'
UNION ALL SELECT 3,'3 month'
)
,
transaction(id,user_id,subscribed_on,plan_id) AS (
SELECT 1,2,DATE '2020-09-04',1
UNION ALL SELECT 2,4,DATE '2019-02-22',2
)
SELECT t.*
FROM transaction t
INNER JOIN plan p ON p.plan_id = t.plan_id
WHERE t.user_id = 2
AND DATE_ADD(
DATE(t.subscribed_on)
, INTERVAL CAST(REPLACE(plan,' month','') AS SIGNED) MONTH
) >= CURDATE()
ORDER BY t.id DESC
(returns no results, as you don't have any dates high enough in your example data...)