OData filter options (handle null or empty values)

We are using $filter system query option in OData to execute filters where the filter value is sent in at runtime.

As an example the OData URL would be like this:

http://services.odata.org/Northwind/Northwind.svc/Customers?$filter=Custo开发者_StackOverflowmerID eq @InCustomerID and Country eq @InCountry

where @InCustomerID & @InCountry are the input values for the equal filter.

At run time when the user enters some value for Customer ID (say 'ABCD') we would replace the @InCustomerID by 'ABCD'

At runtime the query would be as follows:

http://services.odata.org/Northwind/Northwind.svc/Customers?$filter=CustomerID eq 'ABCD' and Country eq 'US'

In the above case the user has entered the following values: CustomerID => 'ABCD' and Country => 'US'

My question is regarding handling of null values in OData $filter. If the user does not enter any value for CustomerID then we want to select all customers from specific country.

In sql case this would be something like:

select * from Customers where ((CustomerID = @InCustomerID) or (@CustomerID is null)) and (Country = @Country).

Essentially how to handle null or empty values so that the specific predicate in the logical condition would always be true.

Does OData filtering enables this option?

You can compare to null using the equality operator like this:

$filter=CustomerID eq null

In your case the query would degenerate to something like:

$filter=(CustomerID eq null) or (null eq null)

Which should work, but it's not very nice. Did you consider removing the predicate completely in such case?