SQL last 6 months visits - sql

Question

Purpose of the report: Identify patients who did not have dental cleanings in the last 6 months
What would be the best approach to write a sql script?
Patients table
patient_id
patient_name
11
Jason Strong
22
Ryan Smith
33
Casey Hammer
Visits table
v_id
patient_id
reason_visit
date_of_visit
1
11
medical
01/01/2021
2
22
dental cleaning
11/10/2020
3
22
annual
01/01/2021
4
11
dental cleaning
5/10/2021
5
11
annual
5/1/2021
Expected
patient_id
patient_name
22
Ryan Smith
33
Casey Hammer
Casey is on the list because she is not in the visits table meaning she never received a cleaning from our office.
Ryan Smith is on the list because it is time for his cleaning.
I was also thinking what if the patient did not have an appointment in the last 6 months but had an future appointment for dental cleaning. I would want to exclude that.

Answer 1

in postgresql:
select * from Patients p
where not exists (
select 1 from Visits v
where v.patient_id = p.patient_id
and reason_visit = 'dental cleaning'
and date_of_visit < now() - interval '6 month'
)
in sql server replace now() - interval '6 month' with dateadd(month, -6,getdate())
in mysql date_add(now(), interval -6 month)