Proper way to handle 'optional' where clause filters in SQL?

Question

Let\'s say you have a stored procedure, and it takes an optional parameter. You want to use this optional parameter in the SQL query. Typically this is how I\'ve seen it don

Answer 1

EDIT: Adding link to similar question/answer with context as to why the union / if...else approach works better than OR logic (FYI, Remus, the answerer in this link, used to work on the SQL Server team developing service broker and other technologies)

Change from using the "or" syntax to a union approach, you'll see 2 seeks that should keep your logical read count as low as possible:

SELECT * FROM dbo.MyTableName t1
WHERE t1.ThisField = 'test'
AND @MyOptionalParam IS NULL 
union all
SELECT * FROM dbo.MyTableName t1
WHERE t1.ThisField = 'test'
AND t1.MyField = @MyOptionalParam

If you want to de-duplicate the results, use a "union" instead of "union all".

EDIT: Demo showing that the optimizer is smart enough to rule out scan with a null variable value in UNION:

if object_id('tempdb..#data') > 0
    drop table #data
go

-- Put in some data
select  top 1000000
        cast(a.name as varchar(100)) as thisField, cast(newid() as varchar(50)) as myField
into    #data
from    sys.columns a
cross join sys.columns b
cross join sys.columns c;
go

-- Shwo count
select count(*) from #data;
go

-- Index on thisField
create clustered index ixc__blah__temp on #data (thisField);
go

set statistics io on;
go

-- Query with a null parameter value
declare @MyOptionalParam varchar(50);
select  *
from    #data d 
where   d.thisField = 'test'
and     @MyOptionalParam is null;
go

-- Union query
declare @MyOptionalParam varchar(50);
select  *
from    #data d 
where   d.thisField = 'test'
and     @MyOptionalParam is null
union all
select  *
from    #data d 
where   d.thisField = 'test'
and     d.myField = '5D25E9F8-EA23-47EE-A954-9D290908EE3E';
go

-- Union query with value
declare @MyOptionalParam varchar(50);
select @MyOptionalParam = '5D25E9F8-EA23-47EE-A954-9D290908EE3E'
select  *
from    #data d 
where   d.thisField = 'test'
and     @MyOptionalParam is null
union all
select  *
from    #data d 
where   d.thisField = 'test'
and     d.myField = '5D25E9F8-EA23-47EE-A954-9D290908EE3E';
go

if object_id('tempdb..#data') > 0
    drop table #data
go