SQL Query with Optional Parameter and Possible Null Column

Question

I am having some difficulty returning all the results I would expect when leaving an optional sql parameter blank on a column that contain null values.

Imagine you have a table with the following (referredby is optional and can therefore be NULL):

Customertable
ID    CustomerName  ReferredBy
1      Aaron         Joe
2      Peter         NULL
3      Steven        Joe

Suppose I want to query with an optional SQL parameter for the referredby field like such:

declare @referredby as varchar(15)

select id, customername
from customertable<br>
where referredby = isnull(@referredby, referredby)

If I leave the parameter null, this would only return:
1 Aaron
3 Steven

How can I use an optional parameter to return all 3 results?

Answer 1

Try This:

select id, customername
from customertable
where (referredby = @referredby OR @referredby is null)

For reasons explained in this post, comparing null = null in sql server returns false (or unknown). As does null != null.

If you really like your syntax I believe you could make it work by setting ansi_Nulls to off: set ansi_nulls off

Answer 2

Add this line on your query:

SELECT ... FROM ... --other codes here
where (referredby = @referredby) OR (@referredby IS NULL)