UNION CTE with ORDER BY on 3 Tables - sql

Question

This is not easy to explain, however, I do believe there is a much nicer way to do what I managed to do and hope to get some help.
I have 3 tables (T1, T2, and T3). I need to get the latest from T1 and T2, then with those results return the results from t3 or from the previous result if t3 is empty. So the LatestDate doesn't really matter if there is a record in t3. Also, if there is no data in t3 and the LatestDate is the same on t1 and t2 (rarely will happen, but want to plan accordingly), I want results from t1.
Here's a sample of what I got, mind you the actual query is has many more fields, but the concept is the same.
CREATE TABLE [dbo].[t1](
[Id] [INT] NOT NULL,
[LatestDate] [DATETIME] NOT NULL
);
CREATE TABLE [dbo].[t2](
[Id] [INT] NOT NULL,
[LatestDate] [DATETIME] NOT NULL
);
CREATE TABLE [dbo].[t3](
[Id] [INT] NOT NULL,
[LatestDate] [DATETIME] NOT NULL
);
INSERT t1 (Id, LatestDate) VALUES (1, CAST(N'2000-01-01T00:00:00.000' AS DateTime));
INSERT t1 (Id, LatestDate) VALUES (2, CAST(N'2001-01-01T00:00:00.000' AS DateTime));
INSERT t1 (Id, LatestDate) VALUES (3, CAST(N'2002-01-01T00:00:00.000' AS DateTime));
INSERT t2 (Id, LatestDate) VALUES (1, CAST(N'2001-01-01T00:00:00.000' AS DateTime));
INSERT t2 (Id, LatestDate) VALUES (2, CAST(N'2002-01-01T00:00:00.000' AS DateTime));
INSERT t2 (Id, LatestDate) VALUES (4, CAST(N'2003-01-01T00:00:00.000' AS DateTime));
INSERT t3 (Id, LatestDate) VALUES (1, CAST(N'2001-01-01T00:00:00.000' AS DateTime));
INSERT t3 (Id, LatestDate) VALUES (2, CAST(N'2000-01-01T00:00:00.000' AS DateTime));
INSERT t3 (Id, LatestDate) VALUES (5, CAST(N'2004-01-01T00:00:00.000' AS DateTime));
GO;
WITH CTE AS (
SELECT TOP 1 * FROM (
SELECT 2 AS Sort, * FROM t1 WHERE t1.id = #UserId
UNION
SELECT 3 AS Sort, * FROM t2 WHERE t2.id = #UserId
) AS t
ORDER BY
t.LatestDate DESC
)
SELECT TOP 1 * FROM (
SELECT TOP 1 * FROM CTE
UNION
SELECT 1 AS Sort, * FROM t3 WHERE t3.id = #UserId
) AS t
ORDER BY
t.Sort;
Expected results:
When #UserID = 1:
Sort Source Id LatestDate
1 t3 1 1/1/2001
When #UserID = 2:
Sort Source Id LatestDate
1 t3 2 1/1/2000
When #UserID = 3:
Sort Source Id LatestDate
2 t1 3 1/1/2002
When #UserID = 4:
Sort Source Id LatestDate
3 t2 4 1/1/2003
When #UserID = 5:
Sort Source Id LatestDate
1 t3 5 1/1/2004
Thanks!

Answer 1

If I understand you correctly you want something like:
DECLARE #UserID INT = 5;
WITH cte AS (
SELECT 1 AS src, 1 as [tbl], * FROM t1 WHERE id = #UserId
UNION ALL SELECT 1, 2, * FROM t2 WHERE id = #UserId
UNION ALL SELECT 3, 3, * FROM t3 WHERE id = #UserId
), cte2 AS (
SELECT * , ROW_NUMBER() OVER(ORDER BY src DESC, LatestDate DESC, tbl ASC) AS rn
FROM cte
)
SELECT Id, LatestDate
FROM cte2
WHERE rn = 1;
RextesterDemo
Using PARTITION BY you could move filtering #userId to final part:
DECLARE #UserID INT = 5;
WITH cte AS (
SELECT 1 AS src, 1 as [tbl], * FROM t1
UNION ALL SELECT 1, 2, * FROM t2
UNION ALL SELECT 3, 3, * FROM t3
), cte2 AS (
SELECT *
,ROW_NUMBER() OVER(PARTITION BY Id ORDER BY src DESC, LatestDate DESC, tbl ASC) AS rn
FROM cte
)
SELECT Id, LatestDate
FROM cte2
WHERE rn = 1
AND id = #UserID
Rextester Demo2

Answer 2

How's this?
This assumes you will take the t3 result if it exists
or the max result from T1 or T2 if not.
if exists(select * from t3 where ID= #ID)
(
select ID, Max(LatestDate), TN='T3'
from t3
where ID= #ID
)
else
(
select top 1 ID, Max(LatestDate) LD,TN
from (Select *,TN='t1' from T1
union all
Select *, TN='t2' from t2) as a
where id=#ID
group by ID,TN
order by LD desc
)