Same query, with or without condition. - sql-server

Question

I want to write a query that selects rows by condition, but if there is no response for that condition, the code should select the same columns, but without the condition.
What the right way to do this? Thanks!
This is my example - select this:
select top 1 *
from tbl
where isActive = 1
but if there is no response, select this instead:
select top 1 *
from tbl
Note that the query is big and complex, so I prefer not to select one and then select the second one, if the first one is null. Also because I have a union after this and it throws an error with this syntax.

Answer 1

Assuming your query is a select top x ... query I would simply use the order by as suggested by Peter's answer. Assuming it's not involving top x, since you wrote your actual query is big and complicated, you can use a common table expression.
The idea is that you encapsulate the big and complex query inside the cte, but instead of writing the where clause to filter out records, you use a case expression to return 1 or 0 if the condition is true or false for each record.
Then you select from that cte where either the case expression results with 1 or there are no records in the cte where the case expression results with 1.
Create and populate sample table (Please save us this step in your future questions)
DECLARE #T AS TABLE
(
Id int identity(1,1),
IsActive bit
)
INSERT INTO #T VALUES
(1),(1),(NULL),(1),(1),(NULL),
(1),(1),(NULL),(1),(1),(NULL),
(1),(1),(NULL),(1),(1),(NULL),
(1),(1),(NULL),(1),(1),(NULL)
The common table expression:
;WITH CTE AS
(
SELECT Id, IsActive,
CASE WHEN IsActive = 1 THEN 1
ELSE 0
END As FoundRecords
FROM #T
)
The query:
SELECT Id, IsActive
FROM CTE
WHERE FoundRecords = 1
OR NOT EXISTS
(
SELECT 1
FROM CTE
WHERE FoundRecords = 1
)
Results:
Id IsActive
1 True
2 True
4 True
5 True
7 True
8 True
10 True
11 True
13 True
14 True
16 True
17 True
19 True
20 True
22 True
23 True
You can see a live demo on rextester

Answer 2

The simplest way is to not use where isActive = 1 but instead order by isActive in descending order:
select top 1 *
from tbl
order by isActive desc
You might even want to consider adding additional ordering fields (e.g. id, name, date), because without that the resulting item could be unpredictable.

Answer 3

You can use the IF-Else condition, right?
DECLARE #result INT
SET #result = CONVERT(INT, (SELECT COUNT(*) FROM tbl WHERE isActive = 1))
IF (#result > 0)
BEGIN
select top 1 * from tbl where isActive = 1;
END
ELSE
BEGIN
select top 1 * from tbl;
END