Add/Skip WHERE CLAUSE based on Condition - sql

Question

I have the below query that takes a TagId list from table variable and returns the list.
But I need to add that CategoryId WHERE condition only if #Tags has the records.
Is it possible to add a WHERE Condition only if my table variable has records otherwise run the same query with 1=1(Always true) and skip the category filter?
DECLARE #TagIdList NVARCHAR(100) = '22,25,47'
DECLARE #Tags TABLE (TagId INT);
WITH CSVtoTable
AS (
SELECT CAST('<XMLRoot><RowData>' + REPLACE(t.val, ',', '</RowData><RowData>') + '</RowData></XMLRoot>' AS XML) AS x
FROM (
SELECT #TagIdList
) AS t(val)
)
INSERT INTO #Tags (TagId)
SELECT m.n.value('.[1]', 'varchar(8000)') AS TagId
FROM CSVtoTable
CROSS APPLY x.nodes('/XMLRoot/RowData') m(n)
SELECT BookingId
,C.CategoryName
FROM Booking B
INNER JOIN Category C ON C.CategoryId = B.CategoryId
WHERE (
b.IsDeleted = 0
OR b.IsDeleted IS NULL
)
-- Add the below where condition only if #Tags has records, else use 1=1
AND C.CategoryId IN (
SELECT DISTINCT CategoryId
FROM CategoryXTag con
WHERE TagId IN (
SELECT TagId
FROM #Tags
)
)

Answer 1

Ultimately you only need to change the end of your query. If performance is an issue you might want to consider using two branches of an if block for each of the two cases even though it's technically possible to squeeze the logic into a single query that doesn't generally optimize as well.
AND
(
C.CategoryId IN (
SELECT CategoryId
FROM CategotryXTag
WHERE TagId IN (
SELECT TagId
FROM #Tags
)
)
OR
(SELECT COUNT(*) FROM #Tags) = 0
)

Answer 2

declare int #tagcount = (select count(*) from #Tags);
SELECT BookingId, C.CategoryName
FROM Booking B
INNER JOIN Category C
ON C.CategoryId = B.CategoryId
AND isnull(b.IsDeleted, 0) = 0
INNER JOIN CategoryXTag con
ON C.CategoryId = con.CategoryId
INNER JOIN #Tags tags
ON tags.TagID = con.TagID
OR #tagcount = 0;
if #tags is empty you might need to put one record in it with a value that would never by used and then or that value
if(#tagcount = 0) insert into #tags values (-100);
or tags.TagID = -100;

Answer 3

You don't need to modify your where clause. Instead, you achieve the same logic by filling #Tags with every TagId from CategoryXTag before running your final query if #Tags is empty after the initial insert:
if ((select count(*) from #Tags) = 0)
insert into #Tags
select distinct TagId
from CategoryXTag;

Answer 4

I'd declare a variable for the #Tags table:
declare #needTagsFilter bit
set #needTagsFilter = case when exists(select 1 from #Tags) then 1 else 0 end
and change the where clause like
AND (
(#needTagsFilter = 0) OR
(C.CategoryId IN (
SELECT DISTINCT CategoryId
FROM CategoryXTag con
WHERE TagId IN (
SELECT TagId
FROM #Tags
)
)
)
COUNT(*) is slower then exists. The downside of adding the count/exists directly to your original query is that SQL server might execute it for all rows.