how to perform date calculations from different tables?

Question

Please forgive me if this is a basic question, I'm a beginner in SQL and need some help performing date calculations from 2 tables in SQL.

I have two tables (patient and chd) they look like this:

Patient :

ID|Age|date         |Alive
--------------------------
1  50  01/09/2013      Y
2  52  11/05/2015      N
3  19  20/07/2016      N

CHD :

    ID|Age|indexdate        
    --------------------
    1  50  01/08/2012    
    2  52  11/11/2013    
    3  19  10/07/2015    

The patient table contains about 500,000 records from 2010-2016 and the CHD table contains about 350,000 records from 2012-2013. What I want to do is see how many CHD patients have died from 2012-2016, and if they have died has 12months passed?

I'm not sure how to do this but I know a join is needed on the ID and we set the where condition with alive as NOT 'Y'

The final output should look like this based on the sample above:

ID|Age|indexdate| deathdate
---------------------------
2  52  11/11/2013 11/05/2015
3  19  10/07/2016 20/07/2016

Any questions let me know!

EDIT: just to make it clear, patients can appear multiple times in the patient table until they die.

Thanks

Answer 1

Let me assume that this query gets the date of death from the patient table:

select p.id, min(p.date) as deathdate
from patient p
where p.Alive = 'N'
group by p.id;

Then, you can get what you want with a join:

select count(*)
from chd c join
     (select p.id, min(p.date) as deathdate
      from patient p
      where p.Alive = 'N'
      group by p.id
     ) pd
     on c.id = pd.id;

You can then address your questions with a where clause in the outer query. For instance:

where deathdate >= current_date - interval '1 year'