Add empty row to query results if no results found - sql

Question

I'm writing stored procs that are being called by a legacy system. One of the constraints of the legacy system is that there must be at least one row in the single result set returned from the stored proc. The standard is to return a zero in the first column (yes, I know!).
The obvious way to achieve this is create a temp table, put the results into it, test for any rows in the temp table and either return the results from the temp table or the single empty result.
Another way might be to do an EXISTS against the same where clause that's in the main query before the main query is executed.
Neither of these are very satisfying. Can anyone think of a better way. I was thinking down the lines of a UNION kind of like this (I'm aware this doesn't work):
--create table #test
--(
-- id int identity,
-- category varchar(10)
--)
--go
--insert #test values ('A')
--insert #test values ('B')
--insert #test values ('C')
declare #category varchar(10)
set #category = 'D'
select
id, category
from #test
where category = #category
union
select
0, ''
from #test
where ##rowcount = 0

Answer 1

Very few options I'm afraid.
You always have to touch the table twice, whether COUNT, EXISTS before, EXISTs in UNION, TOP clause etc
select
id, category
from mytable
where category = #category
union all --edit, of course it's quicker
select
0, ''
where NOT EXISTS (SELECT * FROM mytable where category = #category)
An EXISTS solution is better then COUNT because it will stop when it finds a row. COUNT will traverse all rows to actually count them

Answer 2

It's an old question, but i had the same problem.
Solution is really simple WITHOUT double select:
select top(1) WITH TIES * FROM (
select
id, category, 1 as orderdummy
from #test
where category = #category
union select 0, '', 2) ORDER BY orderdummy
by the "WITH TIES" you get ALL rows (all have a 1 as "orderdummy", so all are ties), or if there is no result, you get your defaultrow.

Answer 3

You can use a full outer join. Something to the effect of ...
declare #category varchar(10)
set #category = 'D'
select #test.id, ISNULL(#test.category, #category) as category from (
select
id, category
from #test
where category = #category
)
FULL OUTER JOIN (Select #category as CategoryHelper ) as EmptyHelper on 1=1
Currently performance testing this scenario myself so not sure on what kind of impact this would have but it will give you a blank row with Category populated.

Answer 4

This is #swe's answer, just reformatted:
CREATE FUNCTION [mail].[f_GetRecipients]
(
#MailContentCode VARCHAR(50)
)
RETURNS TABLE
AS
RETURN
(
SELECT TOP 1 WITH TIES -- Returns either all Priority 1 rows or, if none exist, all Priority 2 rows
[To],
CC,
BCC
FROM (
SELECT
[To],
CC,
BCC,
1 AS Priority
FROM mail.Recipients
WHERE 1 = 1
AND IsActive = 1
AND MailContentCode = #MailContentCode
UNION ALL
SELECT
*,
2 AS Priority
FROM (VALUES
(N'system#company.com', NULL, NULL),
(N'author#company.com', NULL, NULL)
) defaults([To], CC, BCC)
) emails
ORDER BY Priority
)

Answer 5

I guess you could try:
Declare #count int
set #count = 0
Begin
Select #count = Count([Column])
From //Your query
if(#Count = 0)
select 0
else //run your query
The downside is that you're effectively running your query twice, the up side is that you're skiping the temp table.

Answer 6

To avoid duplicating the selecting query, how about a temp table to store the query result first? And based on the temp table, return default row if the temp table is empty or return the temp when it has result?