Can SQL Compare rows in same table , and dynamic select value?

Recently, i got a table which name Appointments

The requirement is that i need to select only one row for each customer by 2 rule:

if same time and (same location or different location), put null on tutor and location.

if different time and (same location or different location), pick the smallest row.

Since i’m so amateur in SQL, i’ve search the method of self join, but it seems not working in this case.

Expected result

Thanks all, have a great day…

Answer

You seem to want the minimum time for each customer, with null values if there are multiple rows and the tutor or location don’t match.

You can use window functions:

select customer, starttime,
       (case when min(location) = max(location) then min(location) end) as location,
       (case when min(tutor) = max(tutor) then min(tutor) end) as tutor
from (select t.*, rank() over (partition by customer order by starttime) as seqnum
      from t
     ) t
where seqnum = 1
group by customer, starttime