[sql] SQL SERVER: Check if variable is null and then assign statement for Where Clause

I am trying to achieve something like the below in WHERE clause in sql.

if (@zipCode ==null)
begin
([Portal].[dbo].[Address].Position.Filter(@radiusBuff) = 1)   
end
else if(@zipCode !=null)
begin
([Portal].[dbo].[Address].PostalCode=@zipCode )
end  

I tried the following:

WHERE ((@zipCode IS NOT NULL AND ([Portal].[dbo].[Address].PostalCode=@zipCode)) OR (@zipCode IS NULL AND ([Portal].[dbo].[Address].Position.Filter(@radiusBuff) = 1)))

which is wrong. Can anyone help in framing the exact statement. Thanks!

is null can be used to check whether null data is coming from a query as in following example:

 declare @Mem varchar(20),@flag int
select @mem=MemberClub from [dbo].[UserMaster] where UserID=@uid
if(@Mem is null)
begin
    set @flag= 0;
end
else
begin
    set @flag=1;
end
return @flag;

is null is the syntax I use for such things, when COALESCE is of no help.

Try:

if (@zipCode is null)
  begin
    ([Portal].[dbo].[Address].Position.Filter(@radiusBuff) = 1)   
  end
else 
  begin
    ([Portal].[dbo].[Address].PostalCode=@zipCode )
  end  

Try the following:

if ((select VisitCount from PageImage where PID=@pid and PageNumber=5) is NULL)
begin
    update PageImage
    set VisitCount=1
    where PID=@pid and PageNumber=@pageno
end 
else
begin
    update PageImage 
    set VisitCount=VisitCount+1
    where PID=@pid and PageNumber=@pageno
end

Try a case statement

WHERE
CASE WHEN @zipCode IS NULL THEN 1
ELSE @zipCode
END