SQL with AND statements, dynamically building up - sql

Question

Aim: I'm building a dynamic SQL string and want to make the search an AND function
Code type: SQL stored procedure within SQL Server Management Studio
Issue: If the first search is not required then I need to know this (I know because the default is '0' in this case. I feel I'm missing a sitter but don't seem to be able to stackoverflow/Google for the solution.
I set up #QueryString with a default of '' so the functionality will work.
What will fix this?:
I've thought about COALESCE and potential use of IF ELSE within the IF but I am hoping there is clean solution along the lines of
SET #QUERYSTRING = IF(#QUERYSTRING = '','', + + ' FIELD1 LIKE ''%' + LTRIM(RTRIM(#s1)) + '%' )
Current example (snippet):
ALTER PROCEDURE [dbo].[spGridSearchTest]
#s1 NVARCHAR(20),
#s2 VARCHAR(20)
AS
BEGIN
DECLARE #QUERY NVARCHAR(MAX) = ''
DECLARE #QUERYSTRING NVARCHAR(MAX) = ''
SET #QUERY = 'SELECT * FROM TblTable'
IF #s1 <> '1234xyz'
SET #QUERYSTRING = #QUERYSTRING + ' Field1 LIKE ''%' + LTRIM(RTRIM(#s1)) + '%'
IF #s2 <> '1234xyz'
SET #QUERYSTRING = #QUERYSTRING + ' Field2 LIKE ''%' + LTRIM(RTRIM#s2)) + '%'
IF LEN(LTRIM(RTRIM(#QUERYSTRING))) > 0
SET #QUERY = LTRIM(RTRIM(#QUERY)) + ' WHERE ' + LTRIM(RTRIM(#QUERYSTRING)) + ''''
EXECUTE(#QUERY)
END

Answer 1

If I understand better your issue:
Try this:
ALTER PROCEDURE [dbo].[spGridSearchTest]
#s1 NVARCHAR(20),
#s2 VARCHAR(20)
AS
BEGIN
DECLARE #QUERY NVARCHAR(MAX) = ''
DECLARE #QUERYSTRING NVARCHAR(MAX) = ''
DECLARE #conditionadded char(1) = 'N'
SET #QUERY = 'SELECT * FROM TblTable'
IF #s1 <> '1234xyz'
BEGIN
SET #QUERYSTRING = ' Field1 LIKE ''%' + LTRIM(RTRIM(#s1)) + '%'
SET #conditionadded = 'Y'
END
IF #s2 <> '1234xyz'
BEGIN
IF (#conditionadded = 'Y')
BEGIN
SET #QUERYSTRING = #QUERYSTRING + ' AND '
END
SET #QUERYSTRING = #QUERYSTRING + ' Field2 LIKE ''%' + LTRIM(RTRIM#s2)) + '%'
SET #conditionadded = 'Y'
END
IF (#conditionadded = 'Y')
BEGIN
SET #QUERY = LTRIM(RTRIM(#QUERY)) + ' WHERE ' + LTRIM(RTRIM(#QUERYSTRING)) + ''''
END
EXECUTE(#QUERY)
END

Answer 2

Do you really need a dynamic query? Why not use something like this:
select
Whatever
from MyTable
where
(#s1 = '1234xyz' or Field1 = #s1)
and
(#s2 = '1234xyz' or Field2 = #s2)
This avoids a security hole, and depending on your query patterns and data set, it might even be faster. And of course, it's pretty easy to read, and you don't have to deal with SQL in strings :)