>>>Terrific! So, we just need to add OR IsNull(ccategory_description) part.
>>>
>>>Haven't thought about it.
>>>
>>>Thanks again for your sharp brain < g >
>>>
>>
>>
>>

>>SELECT Billing_charges.*, Viscodes.ccode_description;
>> FROM ;
>>     Billing_charges ;
>>    LEFT OUTER JOIN VisCodes ;
>>   ON  Viscodes.ccode_value LIKE ( Billing_charges.crevenue_code+"%" );
>>  AND Viscodes.ccategory_description LIKE ( "REVENUE_CODES%" )
>> WHERE  Billing_charges.cbilling_main_fk = ( ?vp_cBilling_main_fk )
>>

>
>These two queries are supposed to be the same. Can you explain the difference between these two? It looks like yours works every time, while the original with IsNull sometimes fails to work. I can not understand it.

Classic error: and ( logical multiplier ) priority!

WHERE  Billing_charges.cbilling_main_fk = ( ?vp_cBilling_main_fk )
AND Viscodes.ccategory_description LIKE ( "REVENUE_CODES%" )
OR Viscodes.ccategory_description IS NULL
* IS
WHERE ( Billing_charges.cbilling_main_fk = ( ?vp_cBilling_main_fk )
AND Viscodes.ccategory_description LIKE ( "REVENUE_CODES%" ) )
OR Viscodes.ccategory_description IS NULL
* CORRECT is
WHERE Billing_charges.cbilling_main_fk = ( ?vp_cBilling_main_fk )
AND (Viscodes.ccategory_description LIKE ( "REVENUE_CODES%" ) 
OR Viscodes.ccategory_description IS NULL)

But even if it works, this last form is not equivalent to mine;
the meaning in relational algebra sense it is different!