Selecting minimal dates, or nulls in SQL - sql

Question

This is grossly oversimplified, but:
I have a table, something like the following:
CREATE TABLE Table1
([ID] int, [USER] varchar(5), [DATE] date)
;
INSERT INTO Table1
([ID], [USER], [DATE])
VALUES
(1, 'A', '2018-10-01'),
(2, 'A', '2018-09-01'),
(3, 'A', NULL),
(4, 'B', '2018-05-03'),
(5, 'B', '2017-04-01'),
(6, 'C', NULL)
;
And for each user, I wish to retrieve the whole row of details where the DATE variable is minimal.
SELECT T.USER FROM TABLE1 T
WHERE T.DATE = (SELECT MIN(DATE) FROM TABLE1 T1 WHERE T1.USER = T.USER)
Works great, however in the instance there is no row with a populated DATE field, there will be a row with a NULL, like the final row of my table above, which I also wish to select.
So my ideal output in this case is:
(2, 'A', '2018-09-01'),
(5, 'B', '2017-04-01'),
(6, 'C', NULL)
SQL fiddle: http://www.sqlfiddle.com/#!9/df42b5/6
I think something could be done using an EXCLUDE statement but it gets complex very quickly.

Answer 1

You may try with row_number()
demo
select * from
(select *, row_number() over(partition by [user] order by [user],case when
[date] is null then 0 else 1 end desc,[date]) as rn
from Table1)x where rn=1

Answer 2

use union and and co-related sub-query with min() function
CREATE TABLE Table1 (ID int, usr varchar(50), DATE1 date)
;
INSERT INTO Table1 VALUES
(1, 'A', '2018-10-01'),
(2, 'A', '2018-09-01'),
(3, 'A', NULL),
(4, 'B', '2018-05-03'),
(5, 'B', '2017-04-01'),
(6, 'C', NULL)
;
select * from Table1 t where
DATE1= (select min(date1) from Table1 t1 where t1.usr=t.usr
) and date1 is not null
union
select * from Table1 t where date1 is null
and t.usr not in ( select usr from Table1 where date1 is not null)
DEMO
ID usr DATE1
2 A 01/09/2018 00:00:00
5 B 01/04/2017 00:00:00
6 C

Answer 3

You can use GROUP BY and JOIN to output the desired results.
select t.Id
, x.[User]
, x.[MinDate] as [Date]
from
(select [User]
, min([Date]) as MinDate
from table1
group by [User]) x
inner join table1 t on t.[User] = x.[User] and (t.[Date] = x.[MinDate] or x.[MinDate] is null)

Answer 4

You can use a Common Table Expression:
;WITH chronology AS (
SELECT
*,
ROW_NUMBER() OVER (
PARTITION BY [USER]
ORDER BY ISNULL([DATE], '2900-01-01') ASC
) Idx
FROM TABLE1
)
SELECT ID, [USER], [DATE]
FROM chronology
WHERE Idx=1;
Using a CTE in this solution simplifies the query improving its readability, maintainability and extensibility. Furthermore, I expect this approach to be optimal in terms of performance.