How to increment count of occurences of column value in MySQL - mysql

Question

I have the following column names:
customer_email
increment_id
other_id (psuedo name)
created_at
increment_id and other_id will be unique, customer_email will have duplicates. As the results are returned I want to know what number of occurance of the email it is.
For each row, I want to know how many times thecustomer_email value has shown up so far. There will be an order by clause at the end for the created_at field and I plan to also add a where clause of where occurrences < 2
I am querying > 5 million rows but performance isn't too important because I'll be running this as a report on a read-replica database from production. In my use case, I will sacrifice performance for robustness.
| customer_email | incremenet_id | other_id | created_at | occurances <- I want this |
|----------------|---------------|----------|---------------------|---------------------------|
| joe#test.com | 1 | 81 | 2019-11-00 00:00:00 | 1 |
| sue#test.com | 2 | 82 | 2019-11-00 00:01:00 | 1 |
| bill#test.com | 3 | 83 | 2019-11-00 00:02:00 | 1 |
| joe#test.com | 4 | 84 | 2019-11-00 00:03:00 | 2 |
| mike#test.com | 5 | 85 | 2019-11-00 00:04:00 | 1 |
| sue#test.com | 6 | 86 | 2019-11-00 00:05:00 | 2 |
| joe#test.com | 7 | 87 | 2019-11-00 00:06:00 | 3 |

Answer 1

You can use variables in earlier versions of MySQL:
select t.*,
(#rn := if(#ce = customer_email, #rn + 1,
if(#ce := customer_email, 1, 1)
)
) as occurrences
from (select t.*
from t
order by customer_email, created_at
) t cross join
(select #ce := '', #rn := 0) params;
In MyQL 8+, I would recommend row_number():
select t.*,
row_number() over (partition by customer_email order by created_at) as occurrences
from t;

Answer 2

If you are running MySQL 8.0, you can just do a window count:
select
t.*,
count(*) over(partition by customer_email order by created_at) occurences
from mytable t
You don't need an order by clause at the end of the query for this to work (but you need one if you want to order the results).
If you need to filter on the results of the window count, an additional level is needed, since window functions cannot be used in the where clause of a query:
select *
from (
select
t.*,
count(*) over(partition by customer_email order by created_at) occurences
from mytable t
) t
where occurences < 2