SQL Server check if value is substring inside isnull

Question

I have a field in UI interface that passes to a stored procedure a null value (when field is unfilled) or a contract number when it is filled. Substrings of the contract number are accepted as input. Inside the procedure, I need to filter the results by this parameter. I need something similar to this:

SELECT * FROM tableName tn
WHERE
tn.ContractNumber LIKE ISNULL('%' + @contractNumber + '%', tn.ContractNumber)

What do you think it is the best approach? Problem is that using a condition like this does not return values.

Answer 1

Simply:

SELECT * 
FROM   tableName tn
WHERE  tn.ContractNumber LIKE '%' + @contractNumber + '%'
    OR @contractNumber IS NULL

You are really checking multiple condition, so having them separated reads more intuitive (for most people, anyway).

I assume this is just a sample query, and you are not selecting * in reality...

Answer 2

Another one:

SELECT * 
FROM   tableName tn
WHERE  tn.ContractNumber LIKE '%' + ISNULL(@contractNumber, '%') + '%'