Question

I need a solution for a conditional WHERE clause in SQL Server. I can’t figure out how to add the functions to the WHERE clause depending on the declared variables. I’ll bracket the conditional parts with the needed logic

DECLARE @distance bigint
DECLARE @photosneeded bigint

SET @distance * 50

SELECT * FROM users u
WHERE
  [IF @distance > 0 THEN distance_function(lat1,lon1,lat2,lon2)<=@distance END]
  AND
  [IF @photosneeded>0 THEN u.photo IS NOT NULL AND u.photo <>'' END]

Answer 1

It can be done using a plain WHERE clause:

SELECT * FROM users u
WHERE
(@distance <= 0 or (distance_function(lat1,lon1,lat2,lon2)<=@distance))
AND
(@photosneeded <= 0 or (u.photo IS NOT NULL AND u.photo <>''))

However you need to be careful with this kind of condition in the clause, because it can really screw up execution plan optimization. See @Iain’s answer for more on why.