How do I execute an If statement inside a CTE - tsql

Question

I need to create a cte based on the value of an SSRS report parameter. The If statement in the attached code executes just fine outside the cte, but errors inside the cte.
I and one of my coworkers have spent several hours unsuccessfully trying to solve this. I have tried using a CASE statement. I have tried placing the WITH cteIngnoreCommID AS inside both the IF and ELSE, but nothing works.
This cte will be joined in subsequent code that creates another cte. Hopefully someone can tell me what I need to do.
--Declare and set Report parameter for testing purposes only
Declare #IgnoreYear as varchar(4)='2010'
;
WITH cteIgnoreCommID as (
IF #IgnoreYear=''
-- create cte with one record have blank values
Select '' as IgnoreComm_ID
,'' as IgnoreYear
,0 as IngnoreBalFwd
ELSE
-- create cte with records obtained from multiple tables.
-- for proof of concept I am just creating one record.
Select '30710000' as IgnoreComm_ID
,#IgnoreYear as IgnoreYear
,5000 as IngnoreBalFwd
)
--The above code run just fine if you comment out the cte lines.
I get the correct results if run the IF/ELSE outside the cte. When run as written with the cte I get the following 2 errors:
Msg 156, Level 15, State 1, Line 5 Incorrect syntax near the keyword
'IF'. Msg 102, Level 15, State 1, Line 13 Incorrect syntax near ')'.

Answer 1

The easiest way would be to use CASE statements inside the CTE. You could modify your CTE to look like this and it should do what you want:
Declare #IgnoreYear as varchar(4)='2010'
;
WITH cteIgnoreCommID as (
Select CASE WHEN #IgnoreYear='' THEN ''
ELSE '30710000'
END AS IgnoreComm_ID
,CASE WHEN #IgnoreYear='' THEN ''
ELSE #IgnoreYear
END as IgnoreYear
,CASE WHEN #IgnoreYear='' THEN 0
ELSE 5000
END as IngnoreBalFwd
)

Answer 2

You can put the if outside the CTE and make the CTE declaration based on the conditional instead:
if 3 > 2
BEGIN
;WITH cte AS (
SELECT 1 AS val
)
SELECT *
FROM cte
END
ELSE
BEGIN
;WITH cte AS (
SELECT 2 AS val
)
SELECT *
FROM cte
END
Using your code:
Declare #IgnoreYear as varchar(4)='2010'
IF #IgnoreYear=''
BEGIN
;WITH cteIgnoreCommID AS (
-- create cte with one record have blank values
Select '' as IgnoreComm_ID
,'' as IgnoreYear
,0 as IngnoreBalFwd
)
SELECT *
FROM cteIgnoreCommID
END
ELSE
BEGIN
;WITH cteIgnoreCommID AS (
Select '30710000' as IgnoreComm_ID
,#IgnoreYear as IgnoreYear
,5000 as IngnoreBalFwd
)
SELECT *
FROM cteIgnoreCommID
END

Answer 3

You could do something like this:
Declare #IgnoreYear as varchar(4)='2010'
;
WITH cteIgnoreCommID as (
-- create cte with one record have blank values
Select '' as IgnoreComm_ID
,'' as IgnoreYear
,0 as IngnoreBalFwd WHERE #IgnoreYear = ''
UNION ALL
-- create cte with records obtained from multiple tables.
-- for proof of concept I am just creating one record.
Select '30710000' as IgnoreComm_ID
,#IgnoreYear as IgnoreYear
,5000 as IngnoreBalFwd WHERE (#IgnoreYear IS NULL OR #IgnoreYear <> '')
)
SELECT *
FROM cteIgnoreCommID
If #IgnoreYear is never NULL, then you can remove the "#IgnoreYear IS NULL OR" part.

Answer 4

I am fairly new to SQL but the answer to my problem ended up being to not create a CTE but a Temp Table. This handled my IF statement fine and I could join to it with the data as required by the report parameter. The code I used to create either a blank temp table or a filled temp table was:
create table #tmpIgnoreComm
( IgnoreComm_ID varchar(8)
,IgnoreYear varchar(4)
,IngnoreBalFwd numeric )
If #IgnoreYear<>''
insert into #tmpIgnoreComm
--(IgnoreComm_ID,IgnoreYear,IngnoreBalFwd)
Select *
From
( Select Distinct
-- Long code to retrieve data
) data
Thanks everyone for you input.