Optional Where Clause Based On Another Table - sql

Question

I have a stored proc where a table of integers is passed as a parameter. I'm trying to find a reasonable way of writing "give me all the records, but if the parameter table has values in it then limit my results to those values".
Both approaches in the queries below work, but when I use either approach in my real-world proc (with a substantial number of joins and apply clauses and a ton of data) it's quite a bit slower than I would like even when the number of rows in the variable table is limited to 1 or 2 records.
Is there a better way of doing this?
-- Apprroach1 - Weird WHERE clause
IF OBJECT_ID('tempdb.dbo.#list') IS NOT NULL DROP TABLE #list
create table #list(Id int)
insert into #list(id) values (726), (712), (725)
declare #listCount int
select #listCount = count(*) from #list
select * from SalesLT.Product p
where 1 = 1
AND
(
#listCount > 0 AND p.ProductID in (select Id from #list)
OR
#listCount = 0
)
and
-- approach 2 - goofy looking JOIN
IF OBJECT_ID('tempdb.dbo.#list') IS NOT NULL DROP TABLE #list
create table #list(Id int)
insert into #list(id) values (726), (712), (725)
declare #listCount int
select #listCount = count(*) from #list
select * from SalesLT.Product p
inner join #list l on
case when #listCount > 0 and l.Id = p.ProductID Then 1
else 0
end = 1

Answer 1

Generally, case when in joins (ON clause) are avoided as it will make less performant query.
Use the left join approach as follows:
select * from SalesLT.Product p
Left join #list l on l.Id = p.ProductID
Where ( (#listCount > 0 and l.id is not null)
or #listCount = 0)

Answer 2

Try this
IF #listCount > 0
BEGIN
SELECT
*
FROM SalesLT.Product p
------------------------
INNER JOIN #list l ON
------------------------
l.Id = p.ProductID
------------------------
END
-- I assume you want to output everything if #listCount = 0
ELSE IF #listCount = 0
BEGIN
SELECT
*
FROM SalesLT.Product p
END
If you have a bunch of joins using that table outputs, you can store the output and use it on your real join/query.
Example:
IF #listCount > 0
BEGIN
SELECT
*
INTO #TempSalesTbl
FROM SalesLT.Product p
------------------------
INNER JOIN #list l ON
------------------------
l.Id = p.ProductID
------------------------
END
-- In your query
SELECT
*
FROM Table A
INNER JOIN #TempSalesTbl ON
...