About SQL statements Group and then sort and then take the first piece of data for each group

Select ranked. Wait Time from
(
SELECT *, Row_number () over (partition by TB. PATIENT_ID ORDER by TB. Wait time ASC) as RowNum
From (select a.patient_id as patient_id, DATEDIFF (Hour,b.admission_date, a.oper_start_date) as wait time
From Pat_operation as a,pat_in_visit as B
where operation_code= ' Setmi_op01 ' and a.patient_id=b.patient_id and discharge_date>= ' 2014-04-01 '
and discharge_date< ' 2014-07-01 ') TB
) ranked
where Ranked.rownum <=1

Because each ID corresponds to two wait times I take the first one

About SQL statements Group and then sort and then take the first piece of data for each group