Workflow Logic for excluding NULL entries for comparison

I am looking for the logic or entry to use to use in the where part of a SQL Table data Assign to exclude NULL entries.

For example:
WHERE
CustomerNumber IS NOT NULL
or
CustomerNumber = 'is empty'

I do not see any reference in the KB or Forums for this.

Craig,
correct SQL syntax to exclude EMPTYs would be

.... where 
CustomerNumber is NOT NULL AND CustomerNumber<>''    ----this is not a double quote, but two single quotes

This is equivalent to Docuware search term in CustomerNumber field NOTEMPTY()