SQL joining on max ID or dates between date - sql

Question

I have the following tables:
Table 1:
with two columns (PatientID,Name)
Table 2:
with four columns (ID,PatientID,FromDate,ToDate)
I need to join (left join) table1 to table2 (on patientid) to get the values in table2 that has getdate() within Fromdate and todate and if there is no such record, then get the latest id.
I am using SQL 2016.
Table 1 Data:
1 Peter
2 Fady
Table 2 data
1 2019-01-01 2019-02-01
1 2019-03-01 2019-04-01
2 2019-06-01 2019-12-01
2 2020-01-01 2020-01-01
I should get:
1 2019-03-01 2019-04-01
2 2019-06-01 2019-12-01

Answer 1

I think apply does what you want. I think you simply want:
select t1.*, t2.*
from table1 t1 outer apply
(select top (1) t2.*
from table2 t2
where t2.patientid = t.patientid
order by fromdate desc
) t2;
I am guessing that you don't have future fromdates. If you do, then the order by can be tweaked to handle this.
EDIT:
If you can have future dates, then this would be tweaked to:
select t1.*, t2.*
from table1 t1 outer apply
(select top (1) t2.*
from table2 t2
where t2.patientid = t.patientid
order by (case when getdate() >= fromdate and getdate < todate() then 1 else 2 end), id desc
) t2;

Answer 2

You can use a temp table. First get the matching data, then update the missing IDs with the Max(Id) as below:
select table1.*, table2.ID
into #temp
from table1 t1
left outer join table2 t2 on t1.PatientID = t2.PatientID
where getdate() between t2.fromdate and t2.todate
update t
set ID = (select max(ID) from table2 t2 where t.PatientID=t2.PatientID)
from #temp t
where t.ID is null
select * from #temp

Answer 3

You can do a union, only one part will have value:
;WITH cte(ID,PatientID,FromDate,ToDate)
AS
(
SELECT t2.ID,t2.PatientID,t2.FromDate,t2.ToDate
FROM Table1 t1
INNER JOIN Table2 t2
ON t1.PatientID=t2.PatientID
WHERE t2.FromDate >=GETDATE() AND t2.Todate<=GETDATE()
),
cte1 (ID)
AS
(
SELEC TMAX(ID) FROM Table2 WHERE NOT EXISTS(SELECT 1 FROM cte)
)
SELECT ID,PatientID,FromDate,ToDate
FROM cte
UNION ALL
SELECT ID,NULL,NULL,NULL
FROM cte1