MySQL 5.7

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
select version();


version()


5.7.35


create table transactions(customer_id integer, purchase_date date);

insert into transactions values
   (1,'2022-01-01'),
   (2,'2022-01-01');
insert into transactions values
   (1,'2022-01-01'),
   (1,'2022-01-02'),
   (1,'2022-01-03'),
   (1,'2022-01-04'),
   (1,'2022-01-05'),
   (1,'2022-01-06'),
   (1,'2022-01-07'),
   (1,'2022-01-08'),
   (1,'2022-01-09'),
   (1,'2022-01-10'),
   (1,'2022-01-11'),
   (1,'2022-01-12'),
   (1,'2022-01-13'),
   (1,'2022-01-14'),
   (1,'2022-01-15'),
   (1,'2022-01-16'),
   (1,'2022-01-17'),
   (1,'2022-01-18'),
   (1,'2022-01-19'),
   (1,'2022-01-20'),
   (1,'2022-01-21'),
   (2,'2022-01-01');
select customer_id, count(*)
from transactions
group by customer_id;


customer_id
count(*)


1
22

2
2


set @r:=1;
select *
from transactions t
inner join (
   select 
      customer_id,
      purchase_date,
      @r:=@r+1 R
   from transactions) x on x.purchase_date=t.purchase_date
                       and x.customer_id=t.customer_id
where x.R=20;


customer_id
purchase_date
customer_id
purchase_date
R


1
2022-01-17
1
2022-01-17
20