Should this T-SQL be done using a UNION - sql-server

Question

Using the table below (call it TableA), I need to create an SQL statement that selects two sets of data and combines them together. First, I need to select those rows where Status = 1 and the DateCreated is greater (meaning newer) than a specified date, that I'll call the StartDate. I also need to select all those rows where Status = 0 and the DateCreated is also greater than specified date BUT where the results are sorted by DateCreated descendingly AND the number of these records is limited to 2.
So if my table data looks like this:
ID Status DateCreated
1 1 2013-05-01 14:00
2 1 2013-05-01 15:00
3 1 2013-05-01 16:00
4 0 2013-05-01 17:00
5 0 2013-05-01 18:00
6 0 2013-05-01 19:00
7 0 2013-05-01 20:00
and I set the #startDate to 2013-05-01 14:30, I want the result set to look like this:
2 1 2013-05-01 15:00
3 1 2013-05-01 16:00
6 0 2013-05-01 19:00
7 0 2013-05-01 20:00
Is this best done with a Union that joins two results or is there a better more efficient way?

Answer 1

You should benchmark with your real data set for performance differences, but just to give you an alternative you can write it using ROW_NUMBER() instead;
SELECT id, status, datecreated FROM (
SELECT id, status, datecreated,
ROW_NUMBER() OVER (PARTITION BY status ORDER BY DateCreated DESC) rn
FROM Table1 WHERE DateCreated > '2013-05-01 14:30'
) a
WHERE status = 1 OR rn < 3
ORDER BY DateCreated;
An SQLfiddle to test with.

Answer 2

No need for UNION - just a WHERE clause translation of your requirements:
declare #t table (ID int not null,Status int not null,DateCreated datetime not null)
insert into #t(ID,Status,DateCreated) values
(1,1,'2013-05-01T14:00:00'),
(2,1,'2013-05-01T15:00:00'),
(3,1,'2013-05-01T16:00:00'),
(4,0,'2013-05-01T17:00:00'),
(5,0,'2013-05-01T18:00:00'),
(6,0,'2013-05-01T19:00:00'),
(7,0,'2013-05-01T20:00:00')
declare #startDate datetime
set #startDate ='2013-05-01T14:30:00'
;With Numbered as (
select *,ROW_NUMBER() OVER (PARTITION BY Status ORDER BY DateCreated desc) as rn
from #t
)
select * from Numbered
where
DateCreated > #startDate and
(
Status = 1 or
Status = 0 and rn <= 2
)
Admittedly, you only need the row numbers for Status 0, but there shouldn't be any harm in running it across all rows.

Answer 3

Try this one -
Query:
DECLARE #temp TABLE
(
ID INT
, [Status] INT
, DateCreated DATETIME
)
INSERT INTO #temp(ID, [Status], DateCreated)
VALUES
(1, 1, '20130501 14:00:00'),
(2, 1, '20130501 15:00:00'),
(3, 1, '20130501 16:00:00'),
(4, 0, '20130501 17:00:00'),
(5, 0, '20130501 18:00:00'),
(6, 0, '20130501 19:00:00'),
(7, 0, '20130501 20:00:00')
DECLARE #startDate DATETIME = '20130501 14:30:00'
SELECT
ID
, [Status]
, DateCreated
FROM (
SELECT
ID
, [Status]
, DateCreated
, ROW_NUMBER() OVER (PARTITION BY [Status] ORDER BY DateCreated DESC) AS rn
FROM #temp
) t
WHERE DateCreated > #startDate
AND (
[Status] % 1 = 1
OR
rn < 3
)
ORDER BY t.DateCreated
Output:
ID Status DateCreated
----------- ----------- -----------------------
2 1 2013-05-01 15:00:00.000
3 1 2013-05-01 16:00:00.000
6 0 2013-05-01 19:00:00.000
7 0 2013-05-01 20:00:00.000