How do I JOIN a CTE with the rest of my query? - common-table-expression

Question

I'm trying to get the first occurrence date in mytable2 and join it with mytable1.
For example:
SELECT userid, occurrence_date
FROM (
SELECT
userid, occurrence_date,
row_number () over (partition by userid order by occurrence_date) rn
FROM mytable2
) cte where rn = 1;
How do I combine that with this query:
SELECT userid, fieldname, event_date, count(transactions)
FROM mytable1
WHERE YEAR(event_date) >= '2020'
GROUP BY userid, fieldname, event_date;
This is my first time working with CTEs, so I don't really understand how to put them together.
FROM mytable1
LEFT JOIN mytable2 ON mytable1.userid = mytable2.userid
here is some sample data:
mytable1
-------------------------------------------------
|userid|fieldname|event_date|count(transactions)|
-------------------------------------------------
| 1 |limes |05/10/2020| 4 |
-------------------------------------------------
| 1 |potatoes |05/10/2020| 3 |
-------------------------------------------------
| 2 |pears |02/15/2020| 8 |
-------------------------------------------------
| 2 |pineapple|03/02/2020| 6 |
-------------------------------------------------
| 2 |oranges |03/05/2020| 10 |
-------------------------------------------------
mytable2
------------------------------------
|userid| occurrence_date |
------------------------------------
| 1 |04/20/2019 01:12:00.000 |
| 1 |04/20/2019 01:12:15.010 |
| 1 |05/10/2020 05:15:33.020 |
| 1 |05/10/2020 05:16:23.011 |
| 2 |03/25/2018 07:33:16.013 |
| 2 |02/15/2020 09:15:30.223 |
| 2 |03/02/2020 11:24:16.210 |
| 2 |03/05/2020 10:30:16.123 |
------------------------------------
mytable3 (result table)... taking occurrence_date from mytable2 and calling it acquisition_date in mytable3 because it is the earliest transaction date.
-------------------------------------------------------------------------
|userid|fieldname|event_date|count(transactions)|acquisition_date |
-------------------------------------------------------------------------
| 1 |limes |05/10/2020| 4 |04/20/2019 01:12:00.000|
-------------------------------------------------------------------------
| 1 |potatoes |05/10/2020| 3 |04/20/2019 01:12:00.000|
-------------------------------------------------------------------------
| 2 |pears |02/15/2020| 8 |03/25/2018 07:33:16.013|
-------------------------------------------------------------------------
| 2 |pineapple|03/02/2020| 6 |03/25/2018 07:33:16.013|
-------------------------------------------------------------------------
| 2 |oranges |03/05/2020| 10 |03/25/2018 07:33:16.013|
-------------------------------------------------------------------------

Answer 1

I figured it out:
with cte AS
(
SELECT *,
ROW_NUMBER() OVER (PARTITION BY userid ORDER BY occurrence_date) AS rn
FROM mytable2
)
SELECT cte.occurrence_date, mytable1.userid, mytable1.fieldname, mytable1.event_date, COUNT(transactions)
FROM mytable1
LEFT JOIN cte ON mytable1.userid = cte.userid and cte.rn = 1
WHERE YEAR(mytable1.event_date) >= '2020'
GROUP BY cte.occurrence_date, mytable1.userid, mytable1.fieldname, mytable1.event_date;