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Question

My current data pull is for customer in 2016. However i want to find out if a customers had a record before 2016.

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select ID,
       name, 
       date_enter,
       Date_leave,
       item_bought,
       lastitmebought =Lag(ID, 1) OVER (PARTITION BY Name ORDER BY Name , Date_leave)
from Customer 
where date between '01/01/2016' and '12/31/2016'

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I know that my query is just looking at customers in 2016. Here is an example of what i would like to do: if a customer A comes in January 1, 2016 and had previously been in the store November,25,2015 i want my lag to give my the ID of the 2015 record as well, not just records found in 2016.

Thank you

Answer 1

If you want to know if a customer has a record before 2016, I would suggest aggregation:

select c.id
from Customer c
group by c.id
having min(date_enter) < '2016-01-01';

If you want the ids of customers that are in both 2016 and 2015:

select c.id
from Customer c
group by c.id
having max(date_enter) >= '2016-01-01' and
       sum(case when date_enter >= '2015-01-01' and date_enter < '2016-01-01' then 1 else 0 end) > 0;