MySQL 8.0

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select version();


version()


8.0.27


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


select *
from transactions t
inner join (
   select 
      customer_id,
      purchase_date,
      row_number() over (partition by customer_id order by purchase_date) 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
2022-01-19
1
2022-01-19
20