Is a T-SQL conditional TOP clause possible? - tsql

Question

I want to dynamically use TOP or not sort of like this...
SELECT #SomeNumber CASE WHERE 0 THEN TOP 5 COLUMNNAME
ELSE COLUMNNAME
END
FROM TABLE

Answer 1

I hope to have understood your problem: you want to select the TOP 5 rows if you pass #SomeNumber = 0 else select all th etable rows
As a first straight implementation you can do something like that
declare #SomeNumber as int
set #SomeNumber = 5
-- set #SomeNumber = 1
SELECT TOP (SELECT #SomeNumber) COLUMNNAME FROM MYTABLE
you can change the parameter value in order to have how many rows you want
Otherwise i suggest you to implement a stored procedure (and maybe you already did that, otherwise you can follow the next steps in order to do it)
CREATE procedure [dbo].[TOPCLAUSE]
-- clause parameter
#SomeNumber as integer
AS
IF #SomeNumber = 0
BEGIN
SELECT TOP 5 COLUMNNAME FROM MYTABLE
END
ELSE
BEGIN
SELECT COLUMNNAME FROM MYTABLE
END
GO
Then you can call
exec [dbo].[TOPCLAUSE] 0
exec [dbo].[TOPCLAUSE] 1
I probably not answered your question but let me know if it helped you

Answer 2

I don't think you can.
You could either use dynamic SQL:
Declare #int int
set #int = 10
exec ('Select top ' + #int + ' * From Customers')
Or you could set rowcount
if (#someNumber != 0)
begin
set rowcount 5
end
select * From Customers
set rowcount 0

Answer 3

I've just used something like this:-
Declare #SQL nvarchar(max), #Params nvarchar(max)
set #Params = N''
Set #SQL = N'SELECT ' + Cast(#SomeNumber as varchar) + ' CASE WHERE 0 THEN TOP 5 COLUMNNAME
ELSE COLUMNNAME
END
FROM TABLE'
exec sp_executesql #SQL, #Params

Answer 4

Short answer is no, not the way you have it.
You can however use IF to test and run a different query:
IF (#SomeNumber = 0)
BEGIN
SELECT TOP 5 ColumnName FROM Table
END
ELSE
BEGIN
SELECT ColumnName FROM Table
END

Answer 5

Two options: conditional SQL or dynamic SQL.
(1) Conditional:
IF #SomeNumber = 0
SELECT TOP 5 COLUMNAME FROM TABLE
ELSE
SELECT COLUMNAME FROM TABLE
(2) Dynamic: build up the query in a varchar() and pass it to sp_execute

Answer 6

Another loophole: make use of subquery's with row_number function
DECLARE #DoTopJN AS bit
SET #DoTopJN = 0 -- or 1
SELECT X.Sequence
X.COLUMNA
--etc
FROM (SELECT ROW_NUMBER() OVER (ORDER BY Y.Code) AS Sequence
,Y.COLUMNA
,Y.COLUMNB
-- etc.
FROM Y) X
WHERE ((#DoTopJN = 0) OR (X.Sequence = 1))

Answer 7

I don't think this is possible because TOP is applied on not just a column but the whole row. You would have to create two different select statements and put them in a IF ELSE construct.

Answer 8

To correct SPE109's code:
DECLARE #SomeNumber INT = 0
DECLARE #SQL nvarchar(max), #Params nvarchar(max)
set #Params = N''
SELECT #SQL = N'SELECT ' + CASE WHEN #SomeNumber = 0 THEN '' ELSE 'TOP ' + CAST(#SomeNumber as varchar) END + ' COLUMNNAME FROM TABLE'
exec sp_executesql #SQL, #Params